How much strength of the Domino tenon joint is from the rounded over sides? The reason I ask is that I want to make custom wider tenons and want to know if I need to make the rounded sides exactly the same radius as the stock ones or it doesn't make difference as the holding power is from the glued opposite wide surface areas of the tenon?
IMHO, I don't think that the sides add much strength. The lion's share is from the top, and bottom.
I base that on the fact that the Domino has wider settings (position 2 and 3) for the cutter.
And it is recommended to cut only the first Domino at the regular width, for alignment, and all the others at a wider width, for ease of assembly.
I don't think Festool would have designed it that way if the integrity of the joint would be compromised by very much.