What skimmer are you using now? Maybe we can help you get some extra oomph out of that.
Also you say you have a sump, but you're running a filtration system. It would be better and safer generally (especially in case of power outages) to run without a filter, with just the live rock and sand. That way you don't have a mass of nitrate sitting still in your water system if the power goes out. And all your waste gets processed more efficiently, and released as gas. The main need for a filter system is if you're a FOWLR or filter-based fish-only tank. Reefs are generally filterless.
A potent skimmer, however, is a real asset.
Maxijet 1200 as a skimmer pump for a 75 would be ok, but I'd prefer a Mag 5 with a more potent skimmer (overpower a skimmer and it will do nothing but spit water) ; but for a return pump in a 75, I'd recommend something above a Mag 9.5, unless there are particular reasons for low flow. Can you explain your system's focus? Reef, type, FOWLR, fish load, etc?