There really isn't much available in the US for the 8 series. The US market is supposed to be under 5000 cars now. Only about 250 - 300 appear to be in the hands of active enthusiasts. Most of them/us are on budgets so $13,000 for just parts for 14 pounds of boost and 450 HP is a bit much for our tastes and wallets!
If you were in Europe there are quie a few places that would gladly lighten your wallet by a significant amout to build what ever you want. (about $26,000 installed for that 14 pounds/450 HP there and only in their shop)
The Big Time modifiers were Dinan ("Look at the web site-if you don't see it we don't have it") and Korman who still lists lots of really $pendy engine mods. There are smaller places but most keep the cars for a LOOONG while, sometimes with little apparent progress. I know of a 6 liter conversion going on for 2 years- with a shop that has done them before.
Look on the front page of Clube E31-way down the right hand side for the tuners.