I had initially posted this prior to shopping for a new DAW, trying to get some ideas on what to expect and what to buy.
Since no one provided any insight, I still want to summarize a few findings that may help others.
Here are the reported latency and round trip times for the VS-700 and the Octa-Capture:
VS-700
48 : 1.1 : 8.7 (4.4 : 4.3)
64 : 1.5 : 9.4 (5.1 : 4.3)
96 : 2.2 : 11.9 (5.6 : 6.3)
128 : 2.9 : 18.3 (7.0 : 11.3)
256 : 5.8 : 28.9 (11.8 : 17.1)
buffer size : buffer latency : round trip (in:out)
These are not dependent on your DAW, it's just the interface (took me a while to undestand).
Where the DAW PC comes into the game is what settings you can actually use without audio drop outs. Now, this is highly system specific and I ran into a major problem that the new beefy DAW didn't come with a suitable graphic card i.e. the graphic driver caused latency spikes that made the audio engine stop after 3 minutes. I ended up swapping the graphic card to an ATI HD 5450, which immediately cured the unexpectedly stopping audio engine.
From then onwards I had good performance down to ASIO buffers of 128 (occasionally 96) samples, but it didn't make me happy yet. Eventually I found a set-up error that makes me wonder why it worked so well in the first place: I'm using a VS-700 and a Octa-Capture synced via VS EXPAND, but had forgotten to set the synchronisation in the VS-700 to DIGITAL 1 (which it has to be according to the manual) making the Octa-Capture the master.
Now that I'm using the correct sync mode these interfaces really ROCK!!! I can record at ASIO buffer sizes of 48 samples without drop outs or glitches for 10+ minutes!!