My L5639 would be 'stable' at 200bclk, as in I could pass 8hrs+ memtest and easily complete burn test, but it would occasionally crash under heavy use despite temperatures remaining under control at around 70C. The best way to crash it was to use repo to sync chromeOS source. You'd expect that to be mostly IO limited, but it was a killer when git starts checking out the branches. Dropping back to 190bclk fixed it, but after a year or so of running at 200 I started dropping to 180 to get stability.
Now I've got an X5675 running at 170bclk, which is ultimately only 650MHz faster than the 200bclk previously 'stable'. I've hardly had to tweak the voltages at all and my repo sync job works just fine. Load temps are around 55C. I also added a second set of RAM - getting all 6 sticks detected at boot has been quite a trial and still doesn't work every time but I think this is inherent in the platform from what I read. I suspect that the CPU could go a lot higher, but the RAM issue is a pain. You can't tell if it's going to work - it'll boot fine 99 times and then lose 2 sticks on the last try.
Triple channel RAM is getting hard to find, so I guess I'll leave it for now.