Wow, that's a sleeper for sure! I wish the ST was just a little quicker. 0-60 in 6.7 is on the slow side and most of the cookie cutter cars and/or minivans, suv's are that fast if not faster. My 2017 Miata ND did 0-60 in 5.9- as well as my stock MazdaSpeed 3. That is more than enough to keep up or get ahead of traffic.
As far as keeping up with traffic, I never have a problem in the Mini. For sure most cars are faster, and if someone next to me at a stoplight is bound and determined to zip off and get ahead of me when the light goes green, so what? Let them waste their gas. But keeping up with traffic is no problem, at least for me. No matter what car you get, there will ALWAYS be someone faster.