Stock cooling solutions is good up to ~800 MHZ, after that you get into overheat territory (not to mention parabolic increase in current draw). The buck and ASIC can handle max frequency but I wouldn't run them unless you can measure temperature and its under 80C on the top of the small heatsink (translates to about 90C ASIC core). Anything past that your reducing the lifetime of the ASIC/buck. This will be even worse if you have a bunch of them together (since they are sucking each others hot air).
At 600Mhz they were fine, but at 796Mhz (at stock pot settings) I thought they were getting really hot, I got not hold my finger against the heatsink for more than maybe 10s (so I guess around 50Centrigrade or so). So in came the fans, that helped a lot. I then went up to 832 (adjusting core and memory by about 1/8th of a turn) and although they got a bit warmer again, it feels totally ok, maybe 40C. max. Tomorrow I will increase to 852Mhz, just to keep somewhat up with the rapidly increasing difficulty. But I don't want to sacrifice lifetime for just a further few percent increase in hashing power.