Like yourself, I have been trying to find the definitive coconut buns for quite awhile, but I haven't had much luck. IMHO, the coconut buns from Kim Moon on Dundas, Aromaz at Brimley/Huntingwood, Furama, and other assorted bakeries around town have been okay but not great.
The very best ones I've ever had were from Po Ma, a small bakery which used to be on Broadview just below Gerrard. They were always fresh, and huge, like about 8 inches long, with maybe twice as much filling as any others I've tried. Like I said, I haven't found any coconut buns as good as Po Ma's since they closed, but I would love to know if the former owners have reopened anywhere else.