Thoughts on going from 19" wheels to 20" wheels

I have a GT PP with some basic suspension mods (Eibach Sportlines, sway bars, BMR cradle lockouts etc.), and I've been thinking about getting some lightweight wheels and new tires when my stock Pirellis are done for. Initially, I looked at the MRR M350s, which I liked, and I could get tires in sizes that would keep my diameter fairly close to stock (I was thinking 285/35 front and 305/35 rear MPSS). When the Michelin 4S tires came out, I noticed that they don't have the same sizes available, and the only 285s and 305s they have for 19" wheels are 30 profile tires, which would be over an inch decrease in diameter. The tires for the 20" wheels in a 285/305 combo would retain a closer-to-stock diameter, which had me considering going with 20" wheels instead.

My car is not a daily driver--it's basically a weekend toy and occasional HPDE car--but I don't really want to have dedicated track wheels as I'd rather just keep one set on the whole time. So I have a couple questions:

1) Which of the two options above would be more preferable? Going with a 19" tire in a smaller diameter, or getting 20" wheels and tires that are closer to the stock diameter? The car has full exhaust and Lund tune, and has the stock 3.73 rear end.

2) What are the downsides, if any, to going from a 19" wheel to a 20" wheel? Obviously the 20s are a little heavier, but they're still quite a bit lighter than stock.

3) What would be the difference in handling characteristics between 19" wheels and 20" wheels, assuming they had tires that gave them similar overall diameters?

4) Excluding handling characteristics, does changing wheels and/or wheel tire diameter affect anything besides the speedo? I read that it may affect ABS, traction control, etc.

Sorry for all the questions, but I'm just trying to educate myself. Thanks in advance for the help!

EDIT: I'm going to post this in the wheel/tire subforum as well. I'd like to get opinions from both groups.

1) If you're hitting peak speeds over about 141-142, the 19's are probably going to force you to use 6th (7000 rpm redline assumed). The 20's might give you 148.

2) Really not a concern until/unless you start time trialing and your time vs other drivers' times start to matter.

3) At the same OD and section width, the 19's would need to be roughly 5 profile points taller. You'd probably be sacrificing a little turn-in response and steering precision with the 19's unless you've got those tires mounted on the maximum width wheels suggested for those tire sizes.

4) I'm pretty sure that your change from staggered to square tire sizing is the only thing that might theoretically affect ABS. But since it's less than a 2% change (and 3% changes are widely agreed to be OK) it probably doesn't matter. Diameter changes of over 5% that don't involve staggered to square or vice versa don't seem to matter at all (at least not up to about 1.0g deceleration that I personally know of).


Norm

I'm going in the opposite direction, 18x10 with 305's on all the corners. In regards to question #4, only time I noticed any ill effects was going with something like pizza cutters on the front.

Thanks for the info, I appreciate it!