I think the OP is talking about whether the concept of placing the heaters in the sump is effective...
The answer is yes... one of the big advantages of sumps is that you can hide equipment in them, heaters will function just fine in the sump provided you have enough flow through it. I've never actually seen a sump that didn't have enough flow to handle the heaters... most people put more than enough flow through them for that.
I've always put heaters in the sump, never had a problem doing that. I agree with the multi-heater philosophy... it's beneficial on multiple fronts... everything flower said, plus, keep in mind that heaters can fail in multiple ways. From a non-catastrophic failure perspective, they can fail either off or on... if a heater fails off, having two heaters slows the fall in temperature and buys you time to take action. If a single large heater fails on, however, it's a far worse situation because it can quickly cook a tank. Therefore two heaters help here as well, as a single heater "on" failure is less likely to heat the tank fast enough that you wouldn't have time to notice.
As far as sizing goes... you're in pretty good shape. Generally I like to go by 5 watts per gallon, going with a little more or less depending on your climate. Also, remember to go by your total system volume, not just what's in the main tank, and to consider that the plumbing and such associated with sumps adds to temperature loss, so if you have lots of plumbing and equipment, upsize a bit more to compensate. If it were me, I'd use (3) 250w heaters on this tank.
