There's some who think the 86 could do well in DSP against the RX8's (street prepared allows cutting of fenders and flaring to fit tires), nobody has bothered to cut one up yet. The last dominant DSP car was an E46 330ci on 18x11" wheels running 315/30/18 Hoosier A7's, that car had 235hp stock and owner claimed 250whp when selling, he is now developing an RX8 with an engine swap for SM. DSP is just bolt on power mods, so you're not getting much better than E85, tune, header, if you wanna go nuts SM is the place to go, engine swaps, FI, the limit is how much power you can actually get to the ground.
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whataboutbob is the only person I know who's put effort into making an FRS as fast as possible for autox, it's not a full SM build but he's got a supercharger and 9.5" wheels and is running Hoosiers last I checked, looks like he still hasn't cut the fenders and is spending more time developing aero as he's gotten more interested in track stuff.
https://www.ft86club.com/forums/showthread.php?t=25212
https://www.instagram.com/robert_stangarone/
imho sure go widebody, buy a set of 11" wheels and 315 Hoosiers
You could save a few bucks and probably have a lot of fun on 295's on 10" wheels if you don't care about being totally competitive, step down another level to 275's on 9's which you might not need to widebody for. Should be a ton of fun! There's an off chance you can't get the NA power to really make use of 315's/11s like the heavier E46 could, idk anyone running an RX8 for that class but you'd probably be ok copying their widths to start with.
ok newb advice because you're asking questions seasoned autoxers don't ask: if you want to go as fast as possible you don't need to flare the fenders yet, buy the standard 17x9 wheels and slap on some 245 Hoosier A7's, make sure you have enough camber to not waste them and go from there.
If you want to be competitive start looking at STX builds, lots of fun to be had there.