if there's something wrong with the car, I'd suggest sorting that before trying to up the power in other ways!
136bhp is VERY low indeed, the n/a 20v engine makes 125 without the turbo, and they run the same compression ratio I believe, so your turbo is hardly adding anything!]
My AGU engine after 130 000 miles made 158bhp in standard form. All should still make standard power irrelavent of mileage really, as Audi were fairly conservative with the figures.
176 after the map isn't TOO bad, but again, it's a good 14bhp down on what's possible.
I'd suggest you get a VAG com scan on the go, and also check for all the obvious little things like boost leaks. When did the car last have a new airfilter and plugs?