Pv=NRT Essentially heat = pressure. The "energy" coming out of the motor, used to spin the hot side of the turbo is made up of heat and pressure. Lose that heat, and you lose some of the pressure that would have otherwise helped spool the turbo. That's it; any cooling of the charge between motor and turbo results in less energy to spin the turbo. That said stock WRX, Forester, Legacy, and STi are all flex pipes. There are just too many occurences of solid UP's leaking or cracking. While there are a few people that have leak free solid UP, we've done at least 50 flex UP's and as far as I know ZERO of the ones we've installed have leaked or failed in any way. Why would you take out a flex joint and replace it with a solid pipe. I can ASSURE you that if Subaru thought a solid pipe would be okay there, they would have done it. It would have cost WAY less for them. For performance (not reliability) you can't have too much heat kept between motor and hot side of turbo. I've written up the calculations repeatedly on why cold intake air is so important. I'll find one and link it, but suffice to say, if you can get cold air into the motor it's MUCH better. Stock is a CAI, a higher flow CAI is good (I only suggest the Injen or APS), the Typhoon with the heat-shield works pretty well. If you can avoid the "hot-air intake" of breathing in 160+ degree air from the engine bay it's much better. SS