2017 Manual Transmission cars get a slight horsepower improvement to 205hp over <'16 which have a peak 200hp. No reliable dyno charts have been produced yet so no idea what the character of that will be, likely unnoticeable to all but the most sensitive of butts, likely entirely replicated by an aftermarket tune and high flow air filter. There will also be a gearing change but that's a tradeoff not a straight performance advantage dependent on the course layout, ultimately not worth more than a tenth of a second or two which most drivers can easily lose on a mistake mistake or two during the run.
The BRZ is getting an option package with 'Brembo brakes' and slightly wider 7.5" wheels which should 'feel' better when paired with slightly bigger 225 tires (it will come with 215 tires. If you're competitive who knows what all this will do for the classing, better to wait a year.
A base FR-S will come in a few pounds lighter than a BRZ due to a lack of options (climate control, leather interior, heated seats, etc.) official figures are on the order of ~20 lbs different iirc.
IMO none of those are worth paying any more for, the money would be better spent in the aftermarket if you're serious about performance. I'd get the cheapest one that fulfills your needs so you can spend more money on event fees. If you're casual imo get seat time now, don't wait, there's a plethora of ways to update the current car to match what we know of the upcoming model. All the important stuff is remaining the same. If you can undercut MSRP I'd bet you'll easily come out ahead $/performance wise by buying a '16 on discount vs waiting for a '17+options.
Good luck.
Edit: Just re-read and caught the casual, not too many mawds aspect of the OP.
Absolutely buy a '16 or earlier, sounds like at the most you'll target building a CS car and end up with putting good tires on it in OE-ish size, TRD lowering springs and Koni/Bilstein struts, changing brake pads/fluid, along with a sway bar or two. Find the color you want, decide if you like the alacantara and HID's or cloth and halogens and drive home happy.