It's never going to be anywhere near as strong as a laminated stem piece. A thin ply cutout has veneer grains pointing every which way but the way that would assure maxumum strength. That's along the centerline of the piece.
And planing plywood is not my favorite passtime. I handplaned four sheets of ¼" plywood for 8:1 scarfs. Never again!

: Regarding the use of inner/outer stems for attaching strip ends, Can any body
: see a problem with utilising the ply bow/stern forms?

: * shape the appropriate entry/exit angles onto the form,
: * use jigsaw to cut off shaped edge,
: * temporaily reattach cutoff (allowing for kerf)
: * Bingo!!

: cheers