You can run stock pipes with no cat. I did for a while. Not really louder than with a cat but a bit deeper sounding. You'll like it. I don't think an underseat K&N will do much if anything for flow; maybe a little better than the stock one when a bit dirty (ie it filters better) but the real advantage is that it is serviceable rather than throw away. The pop on deceleration is due to the pipes, although you could still have it with or without the TORs tune. As TC suggested, check which tune has been put in the bike.