There's not a great deal you can do with remapping as such, maybe 10-15 HP with a good tune-up.
Do get more, you're looking at serious dough on cats, exhausts, cams, maybe pistons and valves.
There are guys out there who have fitted superchargers to their S8s, the most 'famous' being a guy in Brazil, but I can't find any links at the minute.
I think Mike's done a fair bit of looking into this.
Depending on what you're looking for, you might be cheaper swapping yours for a Diseasel V8 D3 and chipping it up, torque-tastic. That was my plan until I couldn't find a decent one in my price range without comedy mileage.
