I too am in the same situation as you. I have a 90 and actually did purchase a Sailfin, but not without a plan for when he gets too large. My LFS is going to start taking oversized fish and selling them out of a 1200 gallon tank. I believe that I will be able to keep him for quite a while. I bought him rather small and have set up my 90 to accomodate as big of a fish as possible. My tank is a FOWLR and is set up to provide maximum swimming room.
Some may think this is a stupid way to own fish (buy them and have to sell them later), but I think it is worth it. As long as you have a plan for him later, I say go for it.
Personally, I like the idea of changing the fish in my tank.