Yeah first and foremost I'd test anything and everything you can first to see if there are issues there, and that includes your temperature and how it fluctuates over time.
Second you say your lights are Kessil 150s and 150W MH pendants. How many are we talking about? The Kessils by themselves I would call accent lighting on any tank that is 24" tall (or more), the 150W pendants, I'm assuming you have a good shiny reflector, well again they're not going to give that much pop. Just to throw out some comparison type of numbers of other reefers, when I ran my 180g tank (4'x3'x2') I had 2-250W MH bulbs with lumenarc pendants and 4-6 T5 bulbs (can't recall the total amount it was a while back) with icecap (or similar) reflectors. Kris (Pacifica), has a ton of Kessils over his tank (I want to at least 7?) but they're the 350 and 360 Kessils now his tank is a bit bigger than yours, but not terribly much, but he has them spread out everywhere.
Third if you're going to upgrade, go to 250W halides, a jump to 175W isn't a huge leap and unless you're really worried about electricity wouldn't be my call on something to do. I would rather have 250W bulbs on 6 hours a day than 175W bulbs on 8 hours a day anyways, and then use supplemental lighting during the other time (in fact on my 180g tank, I want to say I only had my MH bulbs on for about 5 hours a day)