how important is the speed rating?
US GT's came with V rated tires.
Anyone notice any difference in handling if you have changed to a slower speed rating?
Most of the online shops that sell rims have NICE deals on rims and tire packages but the tires are some cheap off brand R rated tires.

Just recently had to fork up 550 for some tires, after talking to the guy he said your car is "V" rated and you DO NOT want to seep below that. So I got some V rated Dunlops i think FM901's. And I know my last tires were not V rated because my ass is glued to the road at all times, its pretty cool because now I can understand how our cars handle so well. My advice is do not drop below factory specifications cause it will damper your cars ride.

Higher speed rating makes your car more stable and better steering at high speed. It also makes your ride a bit harsh due to stronger side wall. Never drive faster than speed rating on your tires.
http://www.tirerack.com/tires/tiretech/general/speed.jsp