i have kuhmo's 245 in the fron and kunho's 275 in the back

 

I would think that 275s on the front of a near stock sixer would look nice but you would notice some slow down.

 

i really have liked my kumho's. i will buy more of their tires.

 

+1 good tires

 

mustangs are not setup to run 245 up front and 275's out back. the suspension and chassis is designed for all four tires the same size. the BMW Z4 Chrysler crossfire and many other european cars are setup to run wider in the rear and narrower up front and they handle great like that.

the reason your steeringis touchy is because the 275's all the way around are very responsive. yes they do tend to track abit on highway grooves made by trafic but nothing that is untolerable.

using 245's in the front and 275's in the back eliminates and chances of proper rotation and handles worse that just running 245's on all fours or 275's on all fours. my cars tended to plow (understeer) on the track using 245's in the front and 275's in the back.

275's all the way around made the car extremely easy to throw around the track because of the predictability of zero understeer and very little oversteer.

if your car tracks or wanders too much with 275's you either need alittle wider rim and or adjust the camber/castor or switch to urethane bushings

another reason you have that problem is using 2 different brands of tires. Kuhmo's are alot harder compound than the nittos and the tread design is different. it would be like running V6 struts in the front and MACHI struts in the rear