I see these two pipes and wonder what the difference is. All I see is the 2660 costs a bit more and the 2652 is easier to find. Can someone explain the difference?

The 2660 is going to give you the best power output combined with a conical manifold, the 2652 will give good all round performance. That said, the 60 will make you run richer as a consequence, and the 52 is more forgiving on fuel. Not sure what the difference b/w the RB 2652 and Nova 2652 is, but I think they are pretty much the same.

Strange I thoughtI read on this forum, the 2660 gives better fuel mileage over the 2652

Hahaha, from what Ive heard the 2660 is an all-out power-pipe! I have the RB 2652 on my T10, and it's powerful, but not overpowering, and it lasts about 6 mins on the right tuning - and that is with the short conical manifold as well. A medium sized one will do me better, but first I have to find it!