I have themt those 18s MR7s are heavy - just to let you know. 27.5lbs to be exact.. if this has any effect on ur decision.
Wow that is heavy. I think the 17" tantrums are 18 lbs each (correct me if I'm wrong). That will be a difference of more than 35 lbs of unsprung rotational weight! You are looking at a power decrease of 7-8whp my friend. That is quite significant.