I gonna guess. Although I don't use VST, but Sonar 7 and a slow Windows computer now, I use the same buffer in the RME Settings program - 1024. I pretty sure your computer could handle VST's more than mine, and it seems alot for the buffer, and I am running 2048 in Sonar, but with 7 stereo audio tracks (wave tracks) I am running 7 (14 total) and it does not go up very much if at all, and I think it will be a bunch of tracks before any CPU or Harddisk is starting to look like it is being taxed to wonder if the tracks are going to play or be recorded. Just my guess but I think it is just the way that Sonar through Windows works with the Windows does work. I find that using headphones output (jacks on the babyface) for recording tracks on the Babyface does not give me any latency when recording the next track or the next track or any of the tracks I have done. Well, perhaps someone else can give a different answer, but it now does not bother me that the Settings-> buffer is so high. (on movies on DVD I was using sample of 128 for my slow computer when watching movies and other music like CD and such but using Windows Media player, I am now just leaving it on samples of 1024 in Settings. I think it is Windows and even if you are using Windows 7, that may not mean that Windows 7 is faster than Windows XP just because Microsoft says it is, it may have more to do because of the different Windows versions and it is made for business and not really anything else with their software - background tasks of Windows checking everything.

I do agree, I have an overclocked Q9550 which has about the same cpu power as an i7 920 and I can easily run at 32 and 64 buffer. (but I have a HDSP PCI RME not USB and use cubase 4).

You could do a simple test project with reaper, the demo is free and has no copy protection, so easy to install and to remove. This will give you a clue whether it´s your pc or sonar that is having these issues.