I’ve found those Planet X 50’s a bit flexy, and stuff like the FFWD’s and cheaper Reynolds are heavy, especially the clinchers.
I’m more thinking along the lines of the OP with his PX SL, changing the wheels from Aksiums to something lighter would make a big difference without the need for a new frame. Unless you want to spend mega money on Enve or the higher end Reynolds, you’d be better with a decent shallow section rim on some decent hubs for the same money. Just my opinion of course, and deep section wheels do look much better…
My biggest revelation came when I switched from SPD’s to look pedals with some stiff carbon shoes – the difference was massive.