Torsion Bars... Best Diameter for a Lowered Street / Autocross car?

I run 1.14” with about the same setup as you with 245/50 sidewall tires.

So I’d go 1.12”

If you are more aggressive/committed autocross with autocross slicks you would go 1.18 or more.

They do not do funky things without “preloading” them. You’re not doing wheel stands, completely taking all the weight off them, with bars like these.