[MRSPONZ]

@SteveMiller
You don't mention it, but how many miles on your struts? It may be time for a new set. Double check your air pressure, especially when you foresee hitting 90mph. And you may want to consider a good set of sticky summer tires and not your all-season tires. Wider wheel / tires on the front will change the "feel" and nimbleness of the car. You may like it, or you may not. Some people say that the OEM size of 185/55R16 makes the car feel "squirrely" at higher speeds, not sure if that's what you mean.
Aero bits and pieces should help with high speed stability.

Lets us know on your progress.

 

[MRSPONZ]

Marginal at best. It hasn't gotten worse, but I really haven't driven the car at high speed all that much lately. Car felt fine at 90mph last week. I am considering putting 215's in the back and 205's in the front but that'll be next year (I know there's a thread on Spyderchat with someone putting 215 and 205 on the OEM 6.5" and 6" rims; need to research that again). Will most likely get a Moroso oil pan first. I'll keep this car for the long haul and I still enjoy going for pleasant drives with the wife. Cheers

Did you mean 215/45r16 in the rear? I don't think anyone makes a 215 tire for 15 wheels. 225 yes, but 7 inch wheels are required. You said earlier that you had S-Drives at one time, how did they compare at 90+ to your current PA31s ?
You could always get a splitter for the front for serious high speed driving.

 

[MRSPONZ]

I found these on tirerack:

VREDESTEIN
SPRINT +Grand Touring Summer

• Size: 215/50R15 88W
• Style: Blackwall

PIRELLI
P700-Z (COLLEZIONE)High Performance Summer

• Size: 215/50ZR15 88Y
• AO Audi, Pirelli Classic Tire
• Style: Blackwall

Expensive for my tastes, but I'd like to find out if they'd fit on my OEM rims.

Both those tires would be nice but expensive tires for the back. I believe they fit best on 7 inch wheels. But I did not see a compatible front tire unless you mix brands.
What you can do and what you should do is all a matter of opinion. I agree with Petrus, stick with OEM sizes. If you want wider in the back go with the 2003-05 7 inch rear wheels. You can keep 6 inch in the front or put the 6.5's in the front. That little wheel "nib/tang" on the rear wheel can be unscrewed.