I think the efflux area was reduced on the one piece at the cost of sightly higher temps but with an increase in thrust. That might also be why the gas turbine builders assocation members found that there was no benefit to smoothing the efflux jet in the rear centre of the turbine wheel on small turbines. Cost was never a consideration to home grown metal bashers but it was amazing how things were done in one way just because thats the way everyone had done it for years. I'll ask the GTBA guys for verification but I know my turbines Chinese or British when upgraded lost the two piece.

Paul