I'm assuming you're running in CAM-T, so the mods required to fit 315s all around would be legal. You will definitely need to flare the front fenders to fit 315s on a Fox. I haven't personally put 315s on a Fox, but it's become a popular mod. I do run 315s on my SN95 cars and I've been happy with the performance, but it's worth noting that both of those cars performed very well with 245s. And, despite running 315s, I have been beaten by cars running 275s at my local events. Weld-on steel flares are relatively inexpensive, fiberglass and carbon fiber flares/fenders are more expensive and require bodywork and paint to fully comply with the "finished exterior" rule in CAM. It can be very expensive, and it might not yield the results you're looking for. The car (suspension) has to be able to get everything out of the 315s. How many suspension mods do you intend to make in the future? How many autocross events have you done? What are your autocross goals?

 

For now I am doing BMR rear control arms with sperical bushings, with cheapo shocks. And in the front just the 94-95 spindles with cheapo struts. Goals of a tubular k member and control arms as well as adjustable shocks and struts. I have done 15+ events in a NA Miata, 2014 gt mustang, focus st, and 1991 MR2 turbo, the Miata was low and stiff with very wide 200tw tires and it drove like it was on rails, I placed top 3 in STS. As far as my goals I would like to be competitive in CAM-T with the fox. Right now I have horrible 15x8 wheels that would fit a 245 tire. I was considering running the smaller tire to learn the car before going wider, and I may still do that, however the wheels I have are definitely the wrong specs and require a 1 inch spacer in the rear. I also don't have a huge budget at the moment so if I am going to buy new wheels would just like to go all out and get what I want for the future.

 

My budget has always forced me to do more with less. I think that's made me a better driver and saved me a lot of money in the long run. If I were in your position, I would wait on the 315s. The rims, tires and flares will cost at least $2,500 (for decent rims and competitive tires). Those huge tries will cover up some issues in your setup, but you won't be able to fully capitalize on the added grip, your car will still have the "factory-installed" Fox Body flaws that impact handling, and you will chew up very expensive tires trying to drive around those flaws. Your spindles are limiting the amount of negative camber you can put in the car, which will also limit your ability to get the most out of the 315s.

In my opinion, that same amount of money could be spent on getting the car more balanced and moving the limit of traction further out. Get the adjustable shocks & struts, a larger rear sway bar, some offset A-arm bushings, CC plates, and make sure every bushing on the car is in good shape. Then remove as much free weight as possible. You might need to add a little to the remaining budget to get a set of rims that fit better and 200TW tires, but I would learn the car on 17x9" or 18x9" rims with 245 or 255 tires (you'll have to see what fits your specific car). You could even use OEM 99-04 GT rims with 245 tires while learning the car.

While you're learning the car, you'll also get a sense of your local competition. You might find that you don't need 315s to beat the CAM-T cars in your area.

 

FWIW, I did exactly what you are anticipating. My '86 currently runs 18x11 CCWs with 315/30s on each corner. I run sn95 rear axles and 96+ front spindles. Flares are all metal and 3.5" wide. It also has the full MM suspension (K, a-arms C/Os on all 4, T/A, PHB, STB, FLSFCs), Cobra brakes all around, 5sp, 3:55s. It also has Recaros and a custom 4-point roll bar. All power accessories, A/C and a big stereo.

 

Wheel specs? I also have SN95 axles

 

As someone who has ran 315s on 10.5" wheels (RT660 and Nankang), I have started to decrease my size as I learn more. Everyone said 315s are too big for anything less than 12" wheels. Yes they will fit on 10.5" and 11" but they are pinched with cause more sidewall flex and sloppy feel. A stretched tire will give better steering feel and handle change of direction better.

I am currently running 295 RE-71RS on 18x10.5 ET12, but have brand new 285 RE-71RS waiting to be installed later this month. I am going down in tire size and the car is just as fast if not faster.

To fit larger wheels/tires you need to look at backspacing, so low offset wheels and/or spacers. But this also screws up your scrub radius. When it comes to suspensions everything is a compromise. Then also need to modify body tub, body panels, sway bars may rub, and fix other rub points with hammer, cutoff wheel, welder, or other means.

To summarize you don't need 315s to be competitive. But if you want them it's a lot of work to fit 315s.

Not a fox but similar suspension and fitment issues:

 

315s square on 18*12s (+20mm). '92, SN96-04 spindles, SN95 FCAs, SN95 axles. Offset is perfect in the rear, needs at least a 12mm up front to clear the coil-over sleeve collar up front. Fender silliness required as well.