I just tried this and after building up to normal speed, the hashrate dropped back down to about 1.3GH/s a few minutes later. I checked the frequency and it was set as expected. The hashrate started increasing again, but quite slowly, so I decided to reflash to the previous firmware for now.
Have you tried it yet?
I've just updated the thread with a 24hr comparison of my previous OC @ 262.5 and the HW rate has drastically reduced.
I can pretty much guarrantee that this will work for all BM1382 boards, it just depends on whether you want to do it. First, run it at a frequency within the bitmain spec (the black numbers in the last table) with the voltage set. The rate will take a while to settle and will at the very least match the spec rate (expected hash in the last table), but more likely beat it (as I have demonstrated). More important though, is the lower HW rate (and again, do not jump at conclusions after the first half hour, let it run for a few hours).
If you feel the HW rate is low enough (even not and just feel like it!), turn up the freq within that voltage range (this is mine through my tests not bitmain's) and you should be OK.
For the 262.5 freq test, the voltage setting is 0750, which is the datasheet voltage for freq 250. Start with 250 and I bet you'll get very few HW errors and a good hash speed, then if you feel like it, turn up the freq and watch your hash-speed increase, with still few HW errors.