There's no reason you couldnt run lower rated tires, they just dont want to sell them to you because of the liability.... as in you buy them, then drive faster then they are rated for and they destroy them selves and you wreck your car.... then you sue them for damages..
If you really want the lower rated tires, Just bring in the wheels and have them mounted + Ballanced. Its what I do with my compitition wheels/tires.
and no, the Speed rateing has No effect on the handling of the car... thats more a tread design and rubber compound issue.