Hello, B328iC.
My Z4 originally came with 18" BBS Style 107. The wheels are nice, but I wanted something smaller and more aggressive, so I decided to put some 17" wheels. I haven't driven it that much to really answer your question as accurately as possible, but I can tell you that the difference in weight between the stock BBS and Konig Hypergrams is dramatic! I assume that the weight reduction would result in faster acceleration, or less work to spin the wheels. (Note that I'm a cyclist and have a basic understanding or rotational mass, but I'm not a physicist either.) The ride does feel more comfy with the bigger tires, but my coilovers are on the stiffest dampening setting. There are a couple of threads on here that talk about switching to 17s, or a track setup. Check them out:
1.
https://www.zpost.com/forums/showthr...=694514&page=2
2.
https://www.zpost.com/forums/showthread.php?t=676629
These two were very helpful in determining the best 17" setup for my Z4.
Thanks!