how much power will a zzp downpipe give a L36 3800??

Nothing until you do other mods to make the car outflow the stock sized piping. ZZP tends to cater to the W body cars. W bodies have many exhaust restrictions, one being the DP and a U bend. Luckily we don't have those issues. Our biggest exhasut restriction is the front exhaust manifold. It'* doubtful that it'* holding a stock motor back though. If anything it'* probably giving you a little backpressure and increasing torque.

thanks

Does ZZP have a DP for pre-2000 Bonnevilles? I think the only DPs for our models are at Hogan Perf, and you have to move up to 3". Which, IMHO, you really can't fully utilize without top heavy mods on an NA engine. In fact there appears to be debate whether 3' pipes are worth it on an SSEI, for anything less than a track/strip-capable car. .

I'm running a 3" on mine, along with a 3" cat. I'm still good with the 2.5" exhaust, but I'm nearing the end of that. Billboost I believe is now out flowing his 2.5"