With the change in diameter of tires you are looking at it won't be a lot, you are talking about 5 mph at 80. You will not get anywhere near the top end for this car at autobahn, although not knowing the shift points of a 4 speed can't say you can't hit the rev limiter on 3rd.

The shorter tires while losing a bit of top end, you should gain a little acceleration but also improve braking, and with the 24 this year saving brakes will be an issue, talking to Roland we were one of the few teams that didn't have to swap pads, and a bunch of the a and b class teams did it multiple times at the 14. Also will lower your center of gravity, cause you know the whole vehicle is lower, reduce roll of the tires which should reduce roll of the body, and have less unsprung weight.

However all of it is really a bunch of pretty minor improvements and changes. The wheel weight is probably the biggest thing out of all of it, larger wheels though do give a little more open space which should help cool the brakes and I think when we had 14s the brake heat effected the tires, and definitely raised the tire pressures.

As for bushings, rubber ones are available everywhere, urethane you may have to actually do the research to find what will fit, I doubt any will be listed as fitting so you might have to shop by dimensions.