There are quite a few things that will blunt the performance of an S4, MAF, Split throttle boot, DV's, F Hose etc. A lot of these things are relatively easy and cheap to sort out, some however are not.. (e.g. Turbo's and CATs). If you are not impressed with the performance of an S4 compared with a 1.8T, there is something wrong with it. I have both and there is no comparison performance wise.
I would read up a bit more about the S4 and what can go wrong with them and be a bit more sure of what your buying. A "cheap" S4 could soon turn into a very expensive S4 as a few people on here have found out!.