I got back a 12-15hp gain from a $700 chip from SAM for my '96 GLE. Mind you I have the non-turbo 10 valve, I wouldn't be surprised if a couple more horses would come out of your 20 valve motor but I don't think very much for the money. Eastern Aerospace sells a Garret 6 psi light turbocharger for the non-turbo 850 but the price and installation is about the cost of trading up to a used T-5 (and the T-5 would still smoke it). Kinda blows, were stuck. I'm just going to tune my suspension and get a crisp ride for a while, let my horrible driving record and insurance cost go down, then buy an 850R!
If there was an easy way to add power through a software tweak, don't you think Volvo would have done it?
OEMs tune their engines for best power/efficiency/emissions output, then dial them back a tiny bit to allow for variances as different engine systems age/deteriorate. These aftermarket chips push the performance envelope right to the ragged edge - more fuel, more agressive ignition advance - which could end up causing your car to fail an emissions test, or trip the check engine light once in a while. And the big thing is cost - you might see a 10-15HP increase, for $500 or more. For half that you can add an exhaust that will add 10-15HP, won't jeopardize emissions, and will sound and look a lot more serious.
Turbo cars can benefit greatly from chip upgrades - it's a simple matter to turn up the boost, trading some engine longevity for more power. Most manufacturers turbo motors are built to take some abuse. NA cars are just a lot harder/more $$$ to tune.
I'd suggest you look for an exhaust, maybe an intake kit - and concentrate the rest of your efforts on suspension enhancements.