You may or may not have issues with clearance. It also depends on the actual section width of the tire, which can vary brand-to-brand.
People with aftermarket wheels (even with optimum offsets) have trouble fitting 245/35/18s without added camber. You wont see or feel any difference in a 235/40/18 over a 245/35/18 from rolling diameter alone. You can easily stray from the stock diameter up to 3% (and sometimes 5% depending on who you talk to) without a problem.
You also, IMO, dont want to create more oversteer by reducing treadwidth in the rear just to "balance" out the car with tire widths.
Where are you seeing this understeer? AutoX? Track? Daily driving? Theoretical only?