I'm running one of these on my WRX. I thought it added some better throttle response and probably did help with spool-up. The only thing you may want to be careful of is that I hear they can suck flat from the intake vacuum when running at high boost levels. I've heard some rumblings like they have fixed it, but you may want to contact Perrin yourself for a confirmation.

Yes, it was a bitch to install too. I also have the Perrin MAF hose, but I doubt you're running the stock airbox.