Thanks for the input!
@dabac: that's what i heard also, however, the biggest losses are at 1st and 5th gear (efficiency as low as 85%?!?!), and I am really interested in the middle range for the most part, the -21% and +26% if i am not mistaken. The +50% top might come as an added bonus since here in the Nethelrands it is mostly flat and a low of 51.7" that I currently have can tackle anything with power and rpm to spare. That being said SA AM comes in mind and as the guy in the link i posted has done, the threads, etc fit perfectly. The problem is that AM is only available used and parts might be hard to come by and also it has no NIG so it might slip if out of adjustment between 2nd and 3rd.
@garage sale GT: by purely looking at the internals, the gear pawls (only one set on both the 3 and 5 spd) are on identical places and also the sprinter and S-RF3 shells have the pawl tracks on the left hand bearing cup. I just came across the Brompton schematics (read list of spare parts with exploded views) and there when talking about SA hubs, the sprinter is mentioned as internals only. My question is purely mechanical, whether threads and shell sizes are identical between the two hubs. I can just order the internals and give it a try but it might prove to be a quite costly experiment if it doesn't work, even provided I could return them.
-Carcinogent
P.S. some links that might shed some more light:
S-RF3 shell
http://www.sjscycles.co.uk/product-S...A599-19209.htm
S-RF3 internals
http://www.sjscycles.co.uk/product-S...X120-19309.htm
Sprinter Shell (36h)
http://www.sjscycles.co.uk/product-S...A486-19182.htm
Sprinter internals
http://www.sjscycles.co.uk/product-B...5-SA-13625.htm