Isn't "one size difference" 5 not 2.5? Are you sure you've gone up enough on the main jets? With my big gun pipe, K&N filter and airbox lid removed, I went from a #165 main to a #180 main on the DS650 and I may be able to go higher. I bought the quiet core spark arrestor along with my big gun pipe, incase I need it. Without the spark arrestor it is loud, but that is to be expected - you probably wouldn't see as much HP gain with the quiet core spark arrestor. I saw a Raptor with a big gun slip on and it was pretty quiet - must of had the spark arrestor in. He said he didn't notice much of a difference in power.

 

Maybe with dual carbs you only go up by half the amount you go up with a single carb, assuming a single cylinder?

 

Yamaha claims 2 sizes richer on the right side. It was set up stock with 140 left and 145 right, so 2 sizes up would be 145 left and 150 right. I hope to ride Sunday and see how it runs.