I know that KO4s are always in demand by the S4 boys. Problem is that, as an S4 owner, if your turbo/s have gone then it doesn't really make sense to repair or replace the KO3s when for not much more you can slot some KO4s in. Having said that, bung an ad up matey. Never know.
So you're converting a twin turbo into a single or are you strapping on two holsetts? From reading on the Audi forums, a set of KO4s on an S4 engine are good for 450-500 with the right pipework and map. Problem is, at that power, heat and airflow will be a massive problem. I know that when I took 5th gear to its absolute max the other day I could physically feel the heat being generated through the footwell on my legs! [smilie=icon_eek.gif]