I think it's telling that there are few RS3 Stage II map dyno graphs around. There are no TTRS or RS3 Stage I vs Stage II videos on Youtube, you'd think if it was that much better someone would have run the numbers and made a flashy video to generate interest.
I've seen some claims of 450bhp but with no chart to back it up. KTM are saying up to 480bhp will be possible with the S-tronic box when this engine is fitted in the Xbow - no idea how modified an engine that will be though or at what cost.
Stage I makes most sense, you're not going to be able to feel or notice a 10-15 bhp difference and at £3.5k that is a lot of money for the extra bit of go. The FMIC would be useful on longer hot track sessions but these days I rarely run more than 8-10 laps at a time for the sake of the other components like brakes and tyres. It's not all about the headline numbers either, my Evo 360 tuned to 400bhp was barely different 90% of the time, the ecutek map gave it much better high rev urge which was useful on track - day to day, on road the difference was minimal.
I like Revo maps, they feel aggressive, but my local dealer has to send the RS3 ECU away for unlocking and I'm not doing that. I'd go for the APR map because that also switches off the 'throttle cut by braking' software, this would give the RS3 more adjustability on track which is where I'd want the gain.