A couple things to clear up.
225/40R18 = 637.2mm diameter or 2001.82mm circumfrence
205/55R16 = 631.9mm diameter or 1985.17mm around
0.8% error, hardly noticable. When plus sizing, you don't actually increase (ideally) the diameter (or circumfrence) of the tire. Larger wheels does not mean larger tire as well. Get this out of your heads! Usually, a larger wheel will weigh more and that is where inertia comes into play. Not the case for Silversvt.
Possibly a softer compound and more surface area could contribute, but the plugs are going to be the biggest difference that he will notice.
