You don`t have to shim to run 20% and you should have to shim to run 30% either......I don`t know how good the .28 is but I have a friend than run the .21 and there fast....I wouldn`t say there the fastest but there no a slouch....I would fatten it a couple hours on the HSN and a hour or 2 on the lsn and baby it for a couple tanks before you start dropping the hammer on it.You want it to get use to the new fuel...If it is running fat when you first start it then there is no need to richen it up...Just baby it for a couple of tanks