I'm using the faster riMac with faster graphics, and a DVI dual 2560 monitor. No issues, except with the stupid mini display port to DVI adapter, which on occasion craps out after sleep (but this occurs on ANY computer). I haven't been building 3D models or gaming, but some PS and Lightroom.
The only issue is ergonomics. For best use, the two monitors have to be at different distances. And my riMac adjusts more effectively to ambient light, which makes the other monitor seem like a set of headlights.
Can't comment on thunderbolt; what issues do you think it has?