I have been asking myself the same thing, as long as it fits in within the factory body work (a wider wheel may be required to take advantage of the extra rubber as well). On some cars, you can tune the understeer with a front alignment and tire pressure (like on my last car, a Viper). I think that although the wider tires on the front should help, it will still be a band-aid... my understanding is that the rear suspension has been designed with understeer in mind... maybe something to do with how weight transfers to the front.
I plan to put larger tires on the front as soon as mine arrives. I've got 245's with 8 1/2 wide wheels all around on my E28 (which weighs 600 pounds less than the new M5) with Dinan stage 4 suspension -- perfectly neutral. I think Dinan has put 265's on the front of supercharged 540's, if not yet an M5.
A tire change could also increase the speed at which oversteer is induced if you can find a stickier tire ...
I read of one M3 owner that found he reduced oversteer by changing from the standard Michelin MXX3's to Bridgestone S02 Pole Position's. Has anyone changed their tires on their M5 to something other than factory issue yet?