I once put an Avon 90/90-19 on the front of my Special. It was noticeably quicker, as I recall. I just don't want to be low enough to drag bottom and crash. (BTDT) Crashing is bad.One other (better?) way to drop height is a smaller diameter lower profile tire.
I'm running 3x18 rims front and rear with 80 series tires, the handling cornering is noticeably improved. Take a look at the madness thread lots of things changed from stock so it's a bit difficult to pick a single item as making the difference.