Sounds like fun:
I believe that should give you more than RS4 equivalent performance, your 3” exhaust compensating for the lack of RS4 cams. You’ll be in the K04 turbo operating range, with the turbine benefitting from the freer exhaust. With a Stage3 ECU, the compressor may lose a little efficiency at higher than RS4 boost (17.4 PSI), but you’ll trade off a minimal amount of top HP for a fatter torque curve below that.
I went the same route with some added intake mods (see my garage) and got around 430 HP out of it at 27 PSI boost. I really liked the K04s for their quick response and smooth power delivery. Of course I was lucky with the Stage 3 ECU from APR.
Knowing how critical it is to match the compressor and turbine performance maps to a given engine, I have always been skeptical of hybrid turbos based on the bigger is better philosophy alone.