OK I think I've got it.
Firstly 100% of the thrust comes from the engine. As some of the other commentators have pointed out (eg Checkboard) if you believe otherwise you only need to turn it off to notice the power drops by more than 9 percent or whatever.
This extremely large pressure differential led to a forward thrust vector is just rubbish. It may be true but is a complete red herring.
I think the truth is if you had the engine running at the same altitude but stationary it would produce half the power. The fact that it is moving at Mach 2 doubles the power. Some of this is attributable to the considerable compression of the air entering the engine and some of it is attributable to the efficiencies added by the exhaust geometry.
What this disguises however is that although you are getting say twice as much thrust as you ought to you are having to spend a load of it overcoming the resistance of the forward movement of the engine. I suggest generating a shockwave in an inlet duct is not dissimilar to the work you have to put into the compressor section of a sub-sonic engine.
It is however an intersting point that as the engine struggles by the increasing altitude it is compensated by the increasing speed.