There is also the question of whether certain mods add any power. Intakes and cat back exhausts, for example, have shown inconsistent results on dynos.
Meth has the effect of increasing the octane when the meth is flowing. Increasing the octane is proven to add power on these cars. Pencilgeek ran race gas with no tune or other changes years ago and gained significant power just from the car adapting and increasing timing. But meth is usually added with one or two nozzles in the intake and that is not as good for dispersion as individual injectors (though some of us remember back to the days of throttle body injection). Vishnu has a Procede with meth injection available avafor these cars and has posted the back to back dyno results and they were impressive -- the tune alone was worth about 10 rwhp and the meth alone was worth about 10 rwhp and together, the two were worth at least 20 rwhp.
E85 is another option that is cheaper than race gas, but not much more widely available. The nearest E85 station for me is about 75 miles, for example. It is also much higher octane, though also less energy so you need about 50% more to make the power, which typically means larger injectors.