Like most plastics, I'm pretty sure you could squeeze an extra cm out of the helmet. One other thing to consider is the width of the opeing in the t-visor. If the DP still has the original visor, dunk the whole helmet in the hottest tap water you can get and give it a stretch. (preferably in the back part) Then when that has cooled, you can get another 5 or 10 mm out of the t-visot opening. Don't go too much or the visor will look really off.