You won't need a heatsink running a 28mm motor. If you need a heatsink you are pushing something too hard.
Any 28mm motor will own up on all the 20mm motors.
And I'm sorry for being too stubborn. I'm just like that.
Maybe you should sell the 18t, get a job, and buy the mini e-revo with some hyperion 35c packs and have a really nice system.