Q50 is definitely an upgrade from a 5 year old G37. Q is more refined in every way and the 3.0T even though has 27 less horsepower feels equally as fast.
Also the fuel economy is better with the new engine.
What you might miss is the stiff steering feel, exhaust note and less body roll during cornering.
The Q however is more quiet and smooth compared to G.
I am on my 2nd Q50. I drove a 2014 Q50 premium for 39 months and didn’t have to fix anything but the tires before I returned my lease. It had some software updates early in its life but once it all got sorted out everything was smooth. I am amazed to hear people complaining about falling bits in interior and such as I had no such issues at all.
I currently drive a 17 premium and its equally good. The fuel economy in 3.0T is better than the 3.7 in 14-15 Qs and I feel the low end torque is much better now so feels much more peppy in city compared to the older engine.
All the Intouch software issues are now resolved and the system works as it should.