I am looking into buying a XFR and would like to know if 305/30/20 size tires are going to fit well. I am planning to put these tires on a 20" by 10" wide mags.
Thanks,
Marco

 

Should fit OK on the rear, could be problems on the front. But I guess you are talking about the rear only.
Assuming you already have 285/30/20 rear tyres, and assuming the new rims are the same offset as what you already have, then the new tires will stick out an extra 10 mm in each direction and fill out the wheel well by an extra 6 mm.
I use this calculator: Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

 

My tires rubbed when the car was fully loaded and cornered hard. I only had that size in the rear though. I would think that they might rub quite a bit in the front.