Upgraded sways aren't expensive. That will make a big dent in the body roll. The Hotchkis sways seem nice. Thinking of getting those myself.
I personally think the tstat idea isn't going to make much difference on a street car. The tstat is a binary switch. It's either open or closed. Once your engine is hot enough that the thermostat is open, it's effectively out of the loop and the engine will operate at its designed temperature range that the cooling system can support given the ambient temperature. Drag racers like it to open as soon as possible to keep the engine as cool as they can until they get their run. But it just delays the inevitable.
There's no inexpensive way to get meaningful power added to your hemi. You can certainly add power by upgrading the heads, cams, exhaust, adding a supercharger, nitrous, etc. But then you're looking at a couple of grand at a minimum, especially if you're not supplying the labor. The little "inexpensive bolt on" things such as a "cold air intake" might add a few horses here and there, but they don't make that much difference in terms of power (makes lots of noise though).
Best,