I'll admit I may be biased, as Spy is one my of favourite classes, but really, reducing the backstab angle would be a huge nerf. Spies have it hard enough as it is with the massive number of anti-spy unlocks; getting rid of sidestabs (and thus, several trickstabs) would drive the spy into the ground.
He's already one of the hardest classes to be effective with against good players, I don't think his job needs to be made any more difficult.
Regarding the sapper, I could see it being a great harassment tool. If you sap before killing the engie, the engie will have a real dilemma over whether to remove the sapper (and risk getting stabbed/killed in the process), or gunning you down first (and reducing his sentry to level 1/2).
All it takes is for the engie to be a few seconds away from the building when you sap it, and whether or not he destroys the sapper your effort will actually be worth something.