Spare tyre for off-roading?

No. No removal of shielding necessary.
Gordon has a very handy spread sheet based tyre size calculator for download on his site.
greenovalexperience.com
More -> documents, info
He's done most of the hard work in determining which tyres fit, and what mods (if any) are necessary to make them fit.

Early vehicles didn't have the lump of chassis protruding behind the front wheels. My brother tried 275/55R20 on his 08 RRS but they scraped too badly with the wheels turned.