There are a few others but they are about the same. Stay away from the SLP unless you have a ram air hood. But not worth it. And the regular SLP is to restrictive compared to other.
PM me if you need more info. Also I could reprogram his PCM to match the mods he has. Pm also if interested!
I use a Thrasher for my GTP with a K&N cone. Tell your friend to check out Club Grand Prix! Your online Grand Prix & G8 resource , there is tons of stuff there regarding GP questions and maybe he could check out the parts for sale section so he doesn't have to pay for a brand new CAI.
I used to own a 2001 GTP. I had a WizAir CAI on the car. The intake sounded awsome and although no dyno time there is a gain. Its the best compliment to the supercharger I did.