Using leaded race gas can ruin O2 sensors if used often. You can get unleaded racing gas though also, I beleive the last I bought was Phillips B25?, 100 or 110 octane unleaded, can't remember.
You should not need race gas for your car til you are running into detonation or very high boost. However, it can be good a preventetive measure. I have run a 50/50 mix when turning up the boost even to levels I know are safe and read safe on the gauges just in case.
I don't know how far the stock fuel system on the SRT-4 can provide fuel before leaning but I would say as an estimate 20psi on the stock setup race gas would be a good safety measure. Being as the intercooler is large, detonation should be minimal but ya never know.