In the Sonar 2017.03 Update, there is support for UWP MIDI in Windows. I want to ask how can I try this feature in action, taking some issues described below into account.
 
The point is that I use Sonar only as a “musical Notepad”—for composing arrangements that are to be played by the live band. I do not produce music with Sonar directly, so I need only MIDI-related features like Piano Roll and Staff View.
 
Therefore I do not need sound card or soft synths other than my onboard Realtek ALC892 and Microsoft GS Wavetable Synth with all the default drivers. The primitive MIDI sound they produce are sufficient for me as a guiding line, and I will hear the true sound not from the PC.
 
So, this is the problem: in Sonar Preferences, if I turn UWP on, the playback is not heard. (Namely, I go to Preferences → MIDI → Playback and Recording → Driver Mode and choose UWP.)
 
Is there a way to toggle it without losing the sound? Before UWP was introduced in Sonar, these settings worked for me:

1. Preferences → Audio → Playback and Recording → Driver Mode: MME (32-bit)
   (if I chose any other option—WDM/KS, WASAPI Exclusive, WASAPI Shared, ASIO—the sound disappeared, too.)
2. In the same place: Share Drivers With Other Programs is turned on.

So is it possible to turn UWP on and keep the sound?

I think you will find for your use, the Uwp midi update will have no benefit.

It seems to be just a new way of communicating midi data between windows and multiple midi hosts, instruments, bluetooth midi devices and so on.

As to why there is no sound if you turn it on i cant really say, im yet to try it myself.

I also think that there will be no benefit from switching to the UWP, but I prefer to explore new features in action, because sometimes they prove to be very helpful.
 
Meanwhile, I seem to find a solution. After turning on the UWP, go to Preferences → MIDI → Devices → Outputs, here check a new device—and the sound is back again!

I've gotten it to work but not correctly. When I enable it and engage the transport, SONAR hangs for about 6-10 seconds before starting to record or playback. Not sure why, but the MME engine works fine so I'll stick with that.
 

Near as I can tell, UWP is just an alternate transport protocol that lets you route MIDI in non-traditional ways, such as using your smart phone to send MIDI to a DAW over Bluetooth. 
 
Bluetooth is not an efficient mechanism for transmitting data, so I'm guessing the 6-10 second lag is due to large MIDI buffers having to be filled before anything is transmitted. Given that the conventional MIDI API is just being used as a wrapper for UWP on a desktop computer and therefore an unnecessary extra layer, I can't imagine any advantage to using it in a desktop DAW - for 99.999% of DAW users.
 
One exception might be if you're developing a mobile app (e.g. a synthesizer for your smartphone) in an emulator and you need it to communicate with SONAR.

Not directly related perhaps, but Korg claim their Bluetooth MIDI keyboards add 'only' 7 milliseconds additional latency to whatever latency the DAW already has.

7ms is neither here nor there when typing, using a mouse etc. but is pretty substantial as far as playing a software instrument is concerned.

True. They are hoping potential buyers will see the "only" qualifier and assume 7 milliseconds is nothing. But I'm guessing the Bluetooth controller market isn't geared toward anyone playing in a Dream Theater cover band. More like the guy who shows up at a gig with an iPhone and one of those plastic things with the colored squares for triggering sequences.
 
Personally, my only experience with Bluetooth was a headset for my phone, and that was not pleasant. It ran my battery down in an hour, was prone to interference and half the time couldn't even establish a connection. Give me real wires, thank you very much. I don't even use cordless mice.
 
It should be noted that UWP isn't just a Bluetooth thing, that's just one of the possibilities it opens up. It could also serve as the transport mechanism for programs running on the same computer, such as virtual MIDI cables. I don't see it taking over that category, though, since UWP is unique to Windows 10 and proprietary solutions already exist that are cross-platform.

Bluetooth is a can of worms you don't want to open in a professional or semi- pro environment ... latency is too high capture any real performance ... and reliability simply sucks ...

I tried to use Eventides H9 control via Bluetooth and it's slow to connect and slow to respond. Via USB you turn it on and it just works.

Simple devices like Bluetooth keyboard used for the tablet PC often don't work and require deleting the driver and rebooting. Happens basically after every 2nd win 10 update. If it works it has periods of double triggers probably caused by some interference.

Bluetooth = headache

I find my Apple bluetooth touchpad works with Macs perfectly 99.9% of the time. It's that 0.1% that means I wouldn't like to rely on Bluetooth in a critical situation.

In my experience cheaper PC equivalents of the Apple touchpad are much less reliable, ditto wireless mice and keyboards. Then again they cost much less than Apple stuff.

The thought of relying on Bluetooth throughout a gig frightens me.

As for proprietary protocols for virtual MIDI cabling, reliable wifi MIDI networking etc. Mac OS has had both for a long time, which is one reason you see more Macs on stages than PCs.

TO the OP. 
MS Wavetable only works under MME mode which is the worst environment to use for Sonar. 
For rough GM playback why don't you use the TTS_1. The sound is much better than MS wavetable and it's super easy to use. 
You can then use WASAPI mode and work without as many issues that MME mode suffers. 
You can see bellow why MME mode is the worst. This is a loop back test. Any overdubs will be way way late in MME mode.