I seem to remember an article in Wooden Boat some years ago comparing scarfs and butt joints reinforced with with a plywood patch and just fiberglass. I can't remember the exact results but I do remember that I was surprised. The butt joints were much stronger than I expected. Will have to see if I can find that.

Got to love the internet!! Issue #86 page 113. Dynamite Payson talks about scarf and butt joints. He says he took strips of 1/4" plywood, added 3" glass tape with polyester resin (not epoxy) on both sides. When breaking the wood over a knee, two pieces were tested and both broke away from the joint. As he pointed out this was polyester too. So obviously the joint was strong than the wood.

He also encourages you to test it yourself.