There is nothing wrong with an s300/s400 setup for your goals. I ran a 369/488 gated eetup before going to a 472/491 nongated and I will tell you first hand the 369 is a limiting factor. The exhaust side chokes ralative short of your goal and will need a gate to keep drive pressure in check. Compressor wheel is rather large for the turbine side and leaves a bit to be desired for spool up. My current 472/491 makes way more power effortlessly and spools just as good as the 369/488 did.
Personally, you already have the piping so drop in a 68mm VGT or 72mm VGT and an s485. You will meat your goals and still retain drivability. If you are set on swapping chargers/piping, a 366/485 will get there too and still drive very well. Even a 464/83/.90 & 485/96/1.1 would do wonders to maintain drivabikity and hit your goal easier than either of the 2 suggestions above.
Just my $0.02...
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