This is a two part complex question.
1. Midi structure vs audio structure of computer setup.
2. Looking for MIDI dynamics control.
 
First off, I'm playing E-drums live from a TD-30 and running MIDI into BFD3. BFD3 can run as either a stand alone or hosted by sonar. My sound card is SSL alpha link and an MX4 card with MADI between them. The default setup is to host BFD3 in Sonar. But I would like to use the MX4's card eq and compression on the drums. to do this with the default setup I need to send 32 channels from Sonar to SSL mix, then send a returning mix of 2 to 32 tracks. Here comes the question. Should I set up BFD3 as a stand alone, rout midi from a sonar track to BFD. Then send BFD audio directly to SSL mix, then send the result to Sonar. This way Sonar does not see the audio till it's ready to record. Less "sling" traffic. I'm not sure if the routing setup of Sonar will work easily with the second setup. but that does allow BFD to grab its own set of ram.
 
Now for the fun question. I would like to find a MIDI plug in that controls MIDI velocity like a compressor, limiter, expander, gate.
Yes there are controls in the BFD software for this over the triggers. But it is limited in it's controls. Why not control velocity of a midi performance like a classy compressor does before converting through a soft synth to audio. The resultant "amount of compression" can be sent to a controller signal to effect the sound however you want.
 
Notes: Round trip reported audio latency 1.8 ms, trigger latency 2.5 ms. A total of 4.3 is just fast enough to work for real time. The MX4 card needs 4 samples to eq, compress, and mix. Only BFD and Mic inputs will run into SSLmix, the real work will happen in sonar with Control-1. Also using V-700 and MCU+2 expanders +C4. The Mackie stuff will get used for SSL mix. Will set up the C4 just like the softube but for the SSL stuff in the MX4 card.
 
I'm just about to upgrade to plat from producer X3. Already paid, just about to upgrade everything for the control-1 from Softube.
 

elegentdrum
 
Now for the fun question. I would like to find a MIDI plug in that controls MIDI velocity like a compressor, limiter, expander, gate.
 

Have a look at the Velocity MFX bundled with SONAR.

elegentdrum
Now for the fun question. I would like to find a MIDI plug in that controls MIDI velocity like a compressor, limiter, expander, gate.

 
Not sure if this works for you, but I do this all the time. 
 
In a midi track in sonar,  open the midi track in PRV (piano roll view) and make sure you are "viewing the controller pane" and it's set to view velocity. Select ALL the notes in PRV , hover your mouse over one note until the cursor changes to something that looks like a dart (or at least that's what it looks like to me). Left mouse click and hold and move your mouse up/down and in the controller pane. You'll see the velocity of ALL the notes scale up/down. As they hit the top (vel=127) they stop and the rest continue to increase. 
 
Basically compressing the velocity up against 127.
 
gabo

elegantdrum is searching for stuff over my head.

I wanted to comment that on a more modest connection, I record live drum performances from a Roland TD-12 into BFD3. It works very well.

I use the main outputs of the TD-12 as audio inputs for monitoring the actual live playing. There's no latency as a result. When I play back the recorded MIDI then the BFD3 sounds perform and it sounds great.

the way I understood the post is that the OP plays live performances with this setup (not just records them) thus he is looking for a real time solution and PRV options won't help ... maybe he can confirm???
 
Being a (mediocre) drummer myself with a similar setup (TD20X to VSTi but currently only for recording purposes) I do wonder, however, why you would want a MIDI tool to adjust your dynamics ... I would try to 2 other things first: (A) adjust the velocity settings/curves for the TD30 and the VSTi (I can have it's own settings) and maybe also the trigger settings of the TD30 and (B) use a "regular" compressor of your liking with lowest possible latency in Sonar to adjust the dynamics ... if you "interfere" with the MIDI data you throw away a lot of the nuances/performance you get from hitting slightly differently and you would end up with a lot less interesting / drum-machine-like performance
 
 

elegentdrum
 
Yes there are controls in the BFD software for this over the triggers. But it is limited in it's controls. Why not control velocity of a midi performance like a classy compressor does before converting through a soft synth to audio. The resultant "amount of compression" can be sent to a controller signal to effect the sound however you want.
 

That's not possible AFAIK. MIDI data does not include the kind of material handling of which  makes a compressor (or any FX) good. Using MIDI you can only compress velocities/volume, whereas a good compressor works in a deeper way. Just like MIDI data doesn't include  any harmonies or such, as the possible harmonies don't exist before the audio is triggered.
If you want classy, controllable results, taking in account the sound of the instrument, with any FX, you'll have to work in audio format.
There are many things that you simply cannot do with MIDI data, because the results the FXs give totally depend on the actual sound used.  That's how I've understood this, which doesn't mean it's 100% correct.
 

Years ago I did have a hardware box that was a MIDI matrix. It had I think 8 in's and 8 outs and you could route anything anywhere. It also had a MIDI compressor that was adjustable, and yes it was for velocity levels. Can't think of the brand at this moment, and I subsequently sold it on ebay. Point is it does exist somewhere.

FreeFlyBertl
I would try to 2 other things first: (A) adjust the velocity settings/curves for the TD30

I was just about to post this. Most MIDI input devices allow you to set the velocity curve and that, essentially, is a zero-attack compressor/limiter/expander for MIDI velocity data.
 
It won't give you the same sort of sound-sculpting freedom that an audio compressor would, but that's not possible with MIDI anyway.

As far as the compressor goes, its just a matter of using the same controls like a real compressor(all dynamics). rather than setting ranges in some menu. And that the controls happen real time. The quality of the sound is another matter. However, in the context of mapping a dynamic range to play a dynamic sound like BFD3, then delicate control over the level can have good advantages. I found an existing Sonar plugin called MIDI FX, Cubase has one that's almost the same. Those plug ins do exactly what I was looking for.
 
Back the hart of the question. Is there a better way to get that done than the following chain:
Triggers-TD30-SONAR-MIDI-FX-(Stand alone BFD3)-SSLmix-Sonar.