Shouldn't hurt anything, because your overall diameter of wheel and tire, will be the same. You may pay a little more for the 16" tires, you may want to check on that. You just end up with a little less side wall.
nah it won't hurt your mileage cuz like blu said, it will be the same diameter with just less side wall...but you will have to get new tires to fit the new 16's...do you want to run the same size tire as what is on there or go bigger while your at it since your gonna have to buy new tires anyway? unless you have ugly the ugly factory steel wheels on there i wouldn't change the tire and wheel unless i was going bigger because you just getting a new wheel with a tire that isn't any bigger...and if you have the 'styled' aluminum wheels then i don't think it would look any better and it wouldn't be bigger