I've printed at 0.4 with a 0.4mm nozzle, with no issues. Makerware just does it. Simplify3d winges that some parameter or other is too big, but if you ignore that it prints fine anyway. So no idea what it's complaining about.

I will say that I only use 0.4 for a quick check print. You will get lamination issues and quite frankly it looks crap.
0.3 is what i use for most things.

There is currently no real way to speed up fff style 3d printing and still maintain acceptable print quality.

With a rep 2 you should be able to print at 0.3 at 70-80mm/s quite happily.

Hollow models are good. If you're using support - redesign untill you don't need supports.
Supports and rafts slow print times down a LOT !

Adding active cooling of the print area to the printer, will let you print faster and cut down on need for supports on overhangs and curves.

Use of pla with active cooling will also allow faster lower quality prints as it warps and shrinks less than abs.

If you can get away with 3% infill, then drop infill to 0% and add an extra shell or two. Shells print much faster than infill and 3-4 shells gives a pretty strong print.
You can bridge a surprisingly large area on a hollow model. So you might be surprised what you can print without infill.