Ok, when I moved "power control settings" slider all the way up to 20% in AMD Vision it finally overclocks, (I didn't try to use this slider before cause I've been trying to increase core clock by small values like +10MHz at first and I didn't think it was necessary to give it higher power) I set it to 1160 and am getting steady 480-490MH/s, sometimes it shows even 520 for a second or two. When I set it to 1200 I started getting some visual glitches, so I think 1160 is enough. Now I've got another question, why can't I lower memory clocks to value that is more than 150MHz below core clock? Right now the lowest I can set it to is 1010MHz and if I go lower than this it sets it to the default 1500MHz.
EDIT: I used MSI Afterburner and managed to lower my mem clock all the way down to 300MHz (lower than that caused glitches) aaand I have steady 520MH/s now @1160 clock
Probably higher mem clock used some of that +voltage I set earlier and my gpu usage jumped from 100% to 65% back and forth which cause also clock jumping from 1160 to default 925 and back. Now everything's steady 1160 clock 300 mem, 100% gpu usage and 520MH/s