Boulder Mark seems to work now, even though I can't think what I might have fixed that would have addressed the problem it was having. It does still crash occasionally, but it usually runs to completion.
So I thought I would post a video of boulder mark running on it to give an idea of how fast the C65GS is. That turns out to be about 500 frames per second in this test, which is very nearly 100x the score of a stock C64. The vacuum-like sound is boulder mark making a noise every frame.
As I have mentioned before, boulder mark is a non-linear benchmark, because the first few hundred frames are much more work to draw.
So it is probably a better comparison between the C65GS and Chameleon, since both are fast enough that the slow frames are only a small fraction of the score.
On that basis, the C65GS is about double the speed of the Chameleon, rather than the 4x that SynthMark would suggest. This makes sense, since SynthMark places a fairly heavy weighting on operations that read from IO and colour RAM, even though they are relatively rare operations. So, depending on the workload, the speed difference is likely to be somewhere between 2x and 4x, if using normal 6502 opcodes.