The 2604/ and 2601 are the same pipe with different logos on them. I am pretty sure that the Max 2602 is the same too. The Novarossi is the 2601, and the RB is the 2604. These pipes give slightly more bottom end than the 2630. The 2630 has a little more peak power, and makes it's power higher in the rpm range than the 2601.

Fuel mileage is very hard to predict for each person. One pipe may give you more power, therefor you are less aggressive on the throttle. Sometimes this can get you better fuel mileage than using a pipe with less power, and runtime because you are always on the throttle sucking the fuel.

The 2630 will also run out of fuel about one lap earlier than the 2601. Everything else being equal. Say 15-30 seconds more with the 2601.