Making sure I have spacers figured out.
 

Let's say that I want to end up with 17x8 rims with 235/40 tires. I'm pretty sure those will fit easily and are about the same height as the stock setup.
For simplicity's sake, I want four identical rims, but I would like all the tires to sit pretty close to the same distance from the fenders.
A 45mm offset works well on the front, but the rear needs a 35mm.
When you put a spacer on, you reduce the offset by the thickness of the spacer, correct? So if you put 10mm spacers on the rear and get four 17/8/45 rims, they should all sit pretty much the same distance from the fenders with no clearance issues even if the car is dropped a little, say 1 inch, and given a little camber to increase the fun factor on the turns but not so much that the tires wear oddly on a DD.

Is this correct?

I know there are members who absolutely hate spacers on general principles: please don't go down that road. I just want to know if this would fit and accomplish my goals.

You need to measure the difference yourself and then figure it out. Yes, spacers do reduce offset which is usually called effective offset (if the offset was +45mm and you put a 15mm spacer then you're down to +30 EO or effective offset). There are a few threads where people would install spacers on stock wheels and most of them usually go for 25mm spacers in the back for flush looks and 15-20mm in the front, again, measure it yourself.

There are loads of internet myths you will stumble upon. IMO and experience, I've used spacers in different sizes for most if not all of the cars I've owned in my life and not one incident has happened where a spacer would crack or bolts would shear bla bla. Just make sure you get hub-centric wheel spacers with the correct bore (56.1mm) for the gt86/frs/brz, make sure it's a good brand like Ichiba and/or H&R (my recommendation). Also, make sure you torque them correctly and do it in a star style when installing them, check on the bolts every month and I assure you, you will have ZERO problems.

17x8" +44 with 235/40-17 tires:
http://www.ft86club.com/forums/showthread.php?t=53200

For a street car, I'd say you can get away with around a 15mm spacer max without rubbing issues.

Not sure how low you will go, but assuming street alignment.

-alex