I'm positive the Mustang 100% stock could easily outrun an E46 M3 on any race track.
In fact, the Mustang GT is just as fast as the the E92 at nearly every track.
You already have the brake pads and tires upgraded, just swap some suspension components under it, maybe a seat and call it done.
Would switching to the BMW be cheaper? Likely, but in order to make it as fast, you would be looking at serious money.
In all honesty, if you are looking for a nice track car, sell the Mustang and buy a C5 Z51 or C5 Z06.
In fact, the Mustang GT is just as fast as the the E92 at nearly every track.
You already have the brake pads and tires upgraded, just swap some suspension components under it, maybe a seat and call it done.
Would switching to the BMW be cheaper? Likely, but in order to make it as fast, you would be looking at serious money.
In all honesty, if you are looking for a nice track car, sell the Mustang and buy a C5 Z51 or C5 Z06.