I would try one thing after the other.Making various changes at once you won't really know what effect each individual mod has done imho.I personally disagree a little with some points mentioned.An 18" wheel is generally heavier but provides more stability also, especially ona highspeed straight and high speed corners.How many cup cars do you see with a 17" wheel nowadays? Most are running 19" or bigger.Obviously wheelbase and width are much larger than a bug which justifies the larger wheels also.Imho a larger wheel provides more overall stability in handling and would trade a couple of kilos for it any day.Remmele's bug runs under a 1:12 minute around the Hockenheim small course (997 Porsche GT3 RS runs 1:11)with 19",2 seconds faster than at the Recaro Tuning Days a few years ago with 18",and he has "only" 300bhp.Also what competition are we talking about? Against other track day cars? Wally's power to weight ratio puts him pretty far up in the field and just judging by the videos he's not really pushing it to the limit. I don't mean that negatively on your driving Wally
For autocross a smaller diameter wheel is the way to go for the sharp and short corners but it's much different on an open track.