You are only as strong as your weakest link. This holds true for exhaust systems. A stock 850 exhaust is around 2.25" in diameter - this is good for the stock turbo and boost setup. If you have more mods planned such as a larger turbo and/or ECU upgrade then investing in a larger internal diameter, freer-flowing exhaust is going to yield not only more HP but it is going to give you better throttle response and most importantly, reduce all that extra turbo lag you're going to get. Getting a freer flowing muffler and larger downpipe isn't going to do you much good if you have 2.25" exhaust piping everywhere else. The bottlenecks will just have shifted from the restrictive stock muffler to the other parts of the exhaust that haven't been upgraded. You should still see a gain with the upgrades you suggested, it's just not an ideal setup. It really is a matter of how much you're willing to spend. Personally, I'd always recommend that you go custom in terms of exhaust - more flexibility, more choice and definitely less money than those pre-fab TME and SuperSprint systems (all good exhaust systems of course, just too expensive IMO).