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

Good choice. I have 1" bars on the car in my avatar ('70 Duster), definitely at looking to go up to 1.14"-ish. I'd recommend running 1" bars on anything mostly stock with 14" or 15" wheels and fat-sidewall tires but when you want to get serious it's not nearly enough. After I put on the current wheel and tire setup and lowered my car (less than an inch in the front, about 2" in the back to make it level as it used to have a rake) the increase in grip was so much that the level of body roll in hard cornering is almost as bad as it used to be with my old tires/wheels before I added sway bars. I can get around a track much faster now but I need to stiffen up the suspension more to get the most out of the tires. Also need to do upper A-arms, I get plenty of caster with offset upper bushings in the stock arms but they're like 9 years old at this point.

For reference my car has the 1" t-bars, offset UCA bushings, poly LCA bushings, boxed LCAs, adjustable strut rods, front and rear Hellwig sway bars and Hotchkiss rear leaf springs. Shocks are lame OE replacements, some nice adjustables might be next on my list but it's a toss-up between shocks and bars honestly. I noticed @BergmanAutoCraft recently started carrying tuned adjustable Fox shocks hmm (!!)