I guess you could use the GTX275 SLI as a comparison. It should be about 5 to 10% max behind the GTX285... and it runs 896MB so it would clearly show a difference if the VRAM made a huge diff.
Just looking at the warhead results at 1680x1050 it doesnt seemt to show much of an improvement, the 2 or 3 fps is most likely due to the faster core,shader,mem on the GTX285 2GB.
Even at the highest resolution the difference is quite small.
That wouldn't tell me what the isolated benefit of VRAM is on these parts, I could guesstimate based on other parts too like the GTX 295 but the whole point of these benchmarks is to test actual hardware differences and not have to guess. Besides the difference in VRAM, the GTX 275 and standard 295 also have less bandwidth and fewer ROPs.
Given this benchmark of the GTX 295 MARS is essentially comparing 2GB GTX 285 in SLI, it would've been nice to see standard 1GB GTX 285 in SLI as the MAR's only real advantage over GTX 285 SLI would be the additional VRAM and the ability to run Quad SLI.