No, dont need to downgear anything, you just run the right supercharger at the right revs so you get the airflow you want and not overspeed it at the top end.
And its the other way round, always blow turbo through supercharger, as per delta s4.

And you get better than v8 power delivery, way better, brings in the turbo out of nothing, so dont really need to go too bananas with the supercharger really, depends on the spec and how you want the power delivery.

If you spec it right you also get mentally low pre-turbo backpressure, meaning you can run rediculous amounts of ignition without det.

Only thing you have to take care of, barring the amount of room, which makes it a near on no-go on most cars, is the charge temps, which are much higher, hence why the S4 used 2 massive intercoolers, rather than the 1 of that size that most car sof that power would use.

Supercharger bypass was only used (IIRC, and contrary to popular internet bullshit) off and for light throttle on that car as engines seem to behave quite strangely when letting off the throttle without it it seems from what testing ive seen people do.

Main problem with any setup like this is when it comes down to it, it soon becomes more bother than its worth, mostly due to lack of room and amount of custom fabrication needed.
I mean, why try and make your turbo 4pot behave like a V8 when itd be cheaper and easier and more reliable to just V8 it anyhow. Or even run ALS.