For reference, I added one full leaf to my MP XHD springs (E body) which took the rate from ~142 to ~172. Measured by placing them on a smooth floor upside down, me standing on them and bouncing a little then a helper measuring height with and without me on them. The added leaf was about $100 at a spring shop. You could DIY by cutting the eyes off a spare main spring and adding it to the pack.

For the aggressive driving you do, agree for sure you need more torsion bar. The green brick ran 1". Also better shocks to go with the bigger bar. These two items are going to pretty much use up your budget I think... and should probably come before the rear springs.