I've built a few sets of sidepipes now and they are difficult to get down to legal noise levels. I think the best I did was on Bernies Cobra and it was about 98dB or so and still ran OK.
The pipes I build are modular. You can pull them apart to repack them or change the baffle type for different aplications.
Here's a baffle in the making with a cat converter. The cats actually muffle a fair bit too.
I've just finished a new set for my own car and I've got a new flange design for the baffles which I reckon is easier to use. Boxhead has got a set on his Cobra with the new baffle design. I'll try to post some decent pics up shortly.
With the swapable baffles I can have a set of civilized baffles for the street with cats and more muffling. Whilst not truly noise legal they will be emission legal and quiet enough that you won't go deaf. With about 30 minutes work I can swap them to a 3" straight through baffle for the track. The baffle design also lets me adjust the collector length to suit my motor whilst maintaining a nice outside apearance.
Cheers