I�m in the same process as you.
I bought the 2 filters and the S2 airbox cover. The �insert� is a rubber sleeve that reduces the air intake (it just fits inside the bigger S2 cover) and simply reduces the amount of air entering thr airbox. You would need this if you don�t want to play around with the fueling. You would be much better to at least get the kev fuel mod, and his O2 controller, depending on which bike model you have. I have�nt had a test ride on mine yet, only fitted the filters yesterday, but already had the fuel mods. Everything ive read says the filters plus fuel mods make an appreciable difference to the running, and performance. I also have an MTC single can. Good luck ! Geoff.