I had weeks with a 5.0 recently and I promise you the m3 is and feels much faster, atleast with DCT vs auto 5.0. Problem with the 5.0 is it has quite tall gears and only 6 gears which make it very hard to stay in the powerband atleast with the auto. It also felt like you were sitting in a rectangular box full of cheap plastic that was really uncomfortable.
DCT made a big difference. Its the difference between sitting in a beautiful race car vs a cheap plastic box.
A manual 5.0 vs manual m3 is probably close in 0.60 and quarter but in real life driving, on the freeway for example, dropping a few gears with dct into 3rd going 75 is wayyyyy faster than the 5.0.
I was extremely disapointed I will tell you that and if you drove an m3 and than a 5.0 I would be shocked if you liked the 5.0. It sounds nowhere near as exotic and refined as the m3 8400 v8