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.
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.