FS Autocross Setup

I have been running FS on street tires and plan on sticking around this class for a while. That said, I am looking for ways to improve the overall handling characteristics of my 2011 GT/CS.

The rule changes for 2014 have been interesting and open up some possibilities.
I am currently running the 19x8.5" GT/CS wheels (Heavy) with the Pirelli PZero Neros 255/40R19 (also Heavy).

With the +/-1" rule this year this opens up the possibility of running 18x8.5 wheels and 18" tires. I have been kicking around the options and there are a number of sub 20lb wheels in this category. 18" tires are also several lbs lighter. If I am doing my math right it is possible to drop about 16lbs / wheel by making this switch. That is a lot of unsprung weight. Also all things being equal, a smaller wheel diameter is going to effectively lower the center of gravity for the car by about 1" between the two setups and acceleration will wind up being just a bit quicker, which would be really nice coming out of turns in exchange for a bit of top speed in each gear.
I am not particularly worried about that due to the course setups we have here in the vegas region being oriented to smaller, lighter, slower cars, I have yet to run into a course here where I can top out second.

So my question to all my fellow AutoX fanatics is this. Good idea? Bad idea?
Also do any of you have experience running anything wider than a 255 on an 8.5 rim? I would really like to get some R-S3 275/40R18 squeezed on there but I don't really know if that is feasible or effective with a big heavy beast like this.

-Static