Guessing at expensive parts like O2 sensors will not only be a waste of money, but isn't likely to solve your problem either. Unless your check engine light's on with a code indicating one slow O2 sensor (for example only), I highly doubt there's anything wrong with your O2 sensors.
We could also guess all day here why your perceived performance is less than desireable but if you're convinced that it's not right, it's my opinion that you should have it checked by someone who's experienced with turbo 850s for another opinion before spending money on parts (short of a small diagnostic fee perhaps).