I'd actually go with the stock bar in the rear and the stiffest, baddest, biggest bar you can find up front. That will actually help plant the inside rear wheel better, and improve front end responsiveness.
I ran the Cobb 35mm (copy of the Hotchkis bar iirc) on full stiff to pretty good effect in BS (CS in 2010). Putting the OEM bar back on would let you stay in BS/CS, assuming you have no other illegal mods.
If you aren't legal for stock class (i.e. some other mods put you in BSP or SSM or something) then I'd drop the rear bar, run about medium stiff up front and let the suspension articulation help you out with minimizing inside wheel spin. You still want a bit of bar up front to help with your lack of spring rate.
Even the factory LSD has problems with inside wheel spin, so I'd save my money and go straight to a clutch-type (OS Giken). You won't be able to stay in stock with the factory LSD anyway, since you can't realistically add cruise control to the base model afaik.
But that's just what I'd do. I like a car that's neutral to slightly loose at turn in, easily rotated on trail braking/throttle on, and otherwise goes where I point it. I'm running on 255mm tires in STR right now and the car's pretty well balanced, with more front-end grip than you'd believe out of relatively skinny street tires.