what kind of handling are you going for? from what i've read, going staggered rim size is not going to make handling much better. and lowering springs will make the ride suffer a little bit.
i'm running 15x4 in front and 15x10s out back. the handling is still worlds better then the previous years mustang i've had.
going with some adjustable coilovers you should be able to make it handle how you want.