I agree with import_nation, plus I'm sure that a lot of other companies will be creating products for it. Invidia currently makes some of the best sounding exhaust systems for the EJ20 and EJ25 for the WRX and STI. If the specs are the same, I would imagine that you could use any of their headers for this car, although I've heard that it was completely redesigned. In either case, there will be many options out there as soon as the cars are available. For the first to buy and those that live near some of the smaller aftermarket companies out there, you may have a good shot at getting some free parts if you're willing to let them borrow your brand new car for a few weeks.
In terms of wheel gap, I'll need to see what the stock offset is before I make that decision/recommendation. If they're anything like the WRX and STI, they run high offsets like 50-55 IIRC and I would go down to 30/35.
Doesn't the exhaust on a BRZ already come to a single point? If so, explain to me how splitting it off just before the axle will increase power.^ Power won't be any different. I'm sure there will be some who will run a single but it'll have to be designed well. Especially the collector area. Plus you'd want to get another rear lip design since there is sort of a cutout for 2 exhausts outlets. At least this isn't like the S2000 where there is a dual exhaust for no reason.
I'm just not sure why you would add 15 pounds to make your car look a little better. It's just an abstract concept for me, sorry.I think the dual exhaust looks a little more sophisticated. Whats a 15 extra pounds? If you are goin all out with weight reduction then go ahead and get the single. But it aint gonna make much of a difference without gutting everthing else.