I will say that I do think that using a 2" will give faster fill times when not on plane, but that also makes for really big shutoff valve / hose / fittings / manifold. You're usually pretty limited on space where the manifold goes. 6 min is the fastest mine will fill, and that's while on plane. It's also pretty close to the rating for the pumps - 1200 gph would theoretically fill a 750 in just under 5 min with no restrictions or head loss. If I'm just idling around, it takes closer to 10 min because the sacks don't all fill at the same rate - the sacks with the most restriction fill more slowly, most likely because the other pumps are pulling a larger share of the intake water. When you're on plane with a scupper, you have positive pressure and plenty of flow in the manifold. For me, though, even 15 min to fill is totally acceptable.
No idea on the thread. I post on a lot of ballast threads.
I used 1" PVC swing check valves. I recently ordered a couple more (to replace the 1 brass one I used originally, and 1 for a spare), and the only place I could find them is here:
http://www.thatpetplace.com/pet/prod/199393/product.web plumbingsupply.com used to carry the threaded ones, but only has the slip connection type now.