i have 150W 20000K over a 15H (also have 2x32W actinics), just about right imo.
i used to run it with VHO's (4x30W URI's, for over 2-years) but i've been running the MH for over a year now. while i still prefer the actinics of the VHO's, the point-source-lighting of the mh really can't be beat for aesthetics. currently, this tank is a soft coral-dominated system (zoos and alcyons).
but i've seen uber-MH setups over other nanos.
lunchbucket (- --) had 400W over his 20H. i think he has 2x250W DE's over his upgraded 30G now (not sure). his was a sps dominated system (as is the new one).
liquid (- NR) had 70W over his 2.5g pico. but i'm not sure if that tank still exists as he's gotten out of the hobby, i believe. gorgeous tank and one of the best picos i've ever seen. this tank was (is?) a zoanthid dominated tank.
just keep the light at a reasonable distance (4"~6"). i wouldn't worry about cooking the tank as much as splash hitting the fixture. there will be more heat put into/onto the tank but you just need to combat that with more fans on the canopy or tank, itself. hth