Thanks for the feedback guys!
Sounds like rim width will not be a big issue. I know the wheels will fit the frame as I have another one of these frames with an even wider rim and it fits fine. H Plus Son 'The Hydra' - 25mm external width, running 28mm tires.
The frame is an FBM Sword SQ from 2015. The tubing is straight gauge chromoly, so it'll be on the heavy side, and I have no idea how stiff it is. But right now the limit to my performance is me, not the bike's weight or stiffness.