I think I can offer some relevant experience to the OP
I went from a 2007 z4 coupe 3.0si to a 2014 cayman S
and from a 2016 Cayman S to a 2018 M240i
if you've never had a porsche before, then yes I would say the move is worth it.
they offer a visceral, precise, connected driving experience that is next level as compared to a bmw
amazing handling, sound, NA throttle response
the only thing you might miss from your 135i is low end torque, but the instant throttle response and sound from an NA engine will more than compensate I suspect...do it!
