The Fabspeed stuff is silly loud...my mate has them on-off his 993 C4 and they are stupidly loud...and not even a particularly nice souind...just noise.
There is a bloke on eBay modifies the standard 996/993 boxes and fits a bypass similar to the PSE, which sounds fantastic.
However if you have a local exhaust place, you can have them fit a 2" bypass pipe to the standard boxes and get the same effect, much cheaper. I think my mate paid £50 for his 993 boxes to be done.
Ideally what you would need is a pair of standard 996 stainless rear boxes to have modified.
If you buy a 996, give me a shout...I have a brand new pair of rear boxes sitting in my garage loft, you can heve them if you want to get the bypasses made.
I also have a modified airbox - which is a nice addition!