i drove a 1m and M3 back to back on the same track. i absolutely loved the 1M - light, super tossable, maybe a bit more fun than the M3. but the M3 was on a whole other level. despite the weight it felt so much more capable and i felt like my limits were much higher with the car. factor in DCT and the gap grows larger.
the 1M performed like a champ on the track, no issues whatsoever, but it was a cool day (around 60-65 F)
i'm not a 1-hater either, i owned a launch-year 135i and loved it, even though i knew it wasn't for everyone. i only sold it to get the M3.