Nasaroc,
You make some interesting points. I'm still not completely convinced by your analysis as I would assume that the Surrey bound passengers from Canary Wharf would currently be using the stopping service via Forest Hill.

I have never seen huge numbers of passengers left on the train after Forest Hill, and I suspect most of these passengers get off at Sydenham and the remaining trains before Croydon.
When the ELL was running (admittedly less frequently than 8tph) I did not see huge numbers changing from ELL to get down to Surrey.
Our figures suggested that less than 4% of passengers from Forest Hill travel to Canary Wharf. Even if 10% of passengers to Surrey who currently use our line are coming from Canary Wharf, I don't think that is a lot of passengers based on the numbers currently travelling on our trains. I also do not expect many people to travel from Shoreditch to Surrey in exactly the same way.
I expect that the vast majority of Surrey passengers who currently use London Bridge will have come from the Northern Line and from central London via the Jubilee Line. I would not expect an extra stop at NXG would change their travelling patterns.

Whilst the "Abrahams Special" does seem quite attractive (to people in Surrey) I don't think that it would lead to the chaos you suggest. In fact I think the people who would most object would be those from Surrey who would not welcome hoards of South Londoners travelling on their trains for one stop to get to NXG. The question is who would benefit most - the 70% of Brockley-Sydenham passengers using London Bridge or the 10% (estimate) of Surrey passengers using the fast line?