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Super Speeder: Just to be clear: NO.
To run in FS class, you have to have the same width WHEELS as what came on it. Tires are a different story... squeeze on anything that will fit.
You can go up or down 1" in diameter (either 19" or 21" dia. wheels). Most of the top guys in the Autocross world went down to a 19" diameter wheel. Same staggered widths as stock (8.5 and 9.5 I believe). BUT, they did squeeze a 275 or even 285 width tire onto all four corners. The fronts were "bulging" out a bit more on front rims as technically that is not the "correct" size tire for that rim, but it did work. It was not technically a "square" setup, as they could not rotate their wheels front to back. To rotate the tires around, you have to get them dismounted/remounted on the wheels.
Note: One of the first reasons they did this was there were more tire options in the 19" size than in 20's. More brands to chose from, and a little less money for the smaller size. (Granted, those light weight wheels more than offset any cost savings, as they are NOT cheap... hence the stock wheels still being on my car). PLus, the 19" wheels ended up with a slightly shorter tire, which provided a better gearing ratio at the lower speeds.
There are rules regarding offset as well, but most of the top guys were running light weight wheels with just enough offset to allow the wider tire up front. The only problems that I've ever read about was the ABS occasionally kicking into what they called "ice mode" where it just wasn't calibrated to have SO much traction. (More tire, plus stickier compound autocross tires, and you're stopping on a dime and making change).
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