It is not totally true that a two piece tailcone produces less thrust.

The design and parrameters of a two piece tailcone are very precise, some would say that correctly designed and made,  is more effecient but many keep to single piece just for the ease of manufacture. 

Shows how far wrong it can be, and how much an effect the tailcone has when I gained 3lb of thrust on mine.

I spoke to a very experienced Turbine manufacturer at the last Jet World Masters and he suggested to make my own tailcone and in crude terms to keep squeezing the dia of the output until I had about 650°c at max revs, and he gauranteed I would have more thrust.  I never done that but he was right as the one I have now gives me about 610 with a big thrust increase. 
I probably could get it even better but I am now getting near the 14lbs claimed and the turbine is running perfect so why bother.