Mugen parts are some of the best quality out there, and their fit and finish is top notch. Their header/exhaust fits like stock and the exhaust is probably the one of the best sounding IMO. You can find better performing parts out there, but as RJ said, its for you to decide. As far as your choices, scrap buying the valvesprings, as they are basically just stock type R springs with the mugen name on them. If you went with the full setup (Mugen intake/4-1 header/twinloop/ECU) you will definitely have substantial gains. The ECU basically a race unit tho, and runs very rich, with a 4700rpm vtec point, also it has to be rewired i believe to run A/C. If you go to honda-tech, search for the Mugen items to get more info and people's input on them.