Actually the hardware should be your only concern, the HF's and Si's share the same wiring, but not engines or computers. The Si engine will bolt up to your tranny, as well as intake and exhaust, the only thing you will have to change is the ECU. If you want the Si tranny, the you will need the hubs, but the intake and exhaust will be so similar, only 1-2 HP would be gained by swapping. The 4 sp stick has taller gears for forced induction, the Si shorter for NA. Just depends on your long term goals. Do a search on dogpile for HF to Si in an EF. You will find more than you need. Good luck. (btw, hmotors has a sweet B16A1 complete for $1150)