For Solo2 - Without a doubt you'll get better times with suspension and tires than any engine mods. Can't comment on Saleen. I've heard good things about the Roush suspension, but it is expensive...and I am biased....it's on my wish list after wheels/tires and brakes.
I ran a "driving event" (road racing with limited passing and no "bump and grind") at Laguna Seca. Higher speeds (up to ~110), not as tight turns as Solo2. The Beast exhibited noticable (and fun and predictable) understeer, moderate roll due to the softer 'vert suspension. (but no sidewall scrub,

running 35 psi on stock Nero's) Best bang-for-the-buck mods for that would have been:
1. Big brakes (which requires 18s),
2. a couple degrees of camber, and
3. fatter or R compound rubber.
I had a few scary moments with the brakes, earned two meatballs :sterb: for going off course with my brake to the floor, once in turn 2 and once at the chicane. I now understand what knock-back is all about...once I started doing the brake tap before scrubbing speed, I found brakes were MUCH more predictable (and I stayed on course).
Man, that's some fun stuff! Can't wait to go again!