Stan it all depends on how wide you go.
You can fit 20s by just rolling the guards as long as you get the right offset and dont go too wide.
What size rims are you thinking of getting ?
My new rims are 18x9 front and 18x10 rear and the fronts have no problem whatsoever, even when I lower it, it should be fine. The rears however are a different story, at the current ride height it is not touching at all, even with passengers in the rear. But the actual rim itself sticks out past the guards so I plan on getting them pumped when I respray the car in a few months time.
So id say determine your rim size and offset first and go from there, you may only need to do the rears.
As Adam said before, GTR front guards will be a hassle as you may have issues with the front bar etc and also the way the bonnet lines up. If thats the case you will not only need the GTR guards, but front bar and GTR bonnet. Unless you plan to go wider than 9 inches at the front than dont bother