There's a brand new 2011 Tundra LT with the ACE 4 stroke for sale near here for $8,200. The same dealer is advertising the same sled with the 550 for 7 grand. Don't know where you are in AK but I'm sure shipping adds a pile...
If the MPG difference is substantial over the 550 the extra $1,200 for the 4 stroke will be payed back in fuel savings quickly.