While the MCX is no super-powerful engine it should get the job done for you with the correct prop. We need to know what props you have tried.
WillyT is right, not many riders would need that much weight.
My setup now is (2) 400# sacs in rear lockers (soon to be 720's), full KGB and 750# sac in front walkway.
I don't use the rear hard tanks at all.
Jeff Barton, Keith Lyman and Josh Palma seem to like it fine.