I've had a 335i, 435i Grand Coupe, F80 M3 and now a 2018 M550. The M340i was on my list to test drive but after driving the M550 I didn't even give it a chance.
Like many have stated you don't really get the sensation of speed until you see the speedometer. Crazy fast for the size of car and the ride is so smooth. Tons of room and creature comforts although mine didn't come with Apple CarPlay..But I've gotten over that
If you haven't already, give the m550 a spin, I don't think you'll be disappointed.