What plane is the engine in? Is there a pipe or is it open. A truer test would be without a pipe and the best test would be off the plane and on a bench. However for a comparison of tail cones it is fair.

With the Wren engines they suggested on the early ones to narrow the exhaust to produce more thrust but warned of a rise in temps. Seeing that your engine has higher temps with the single piece suggests that it is more restrictive than the two piece cone. If you could match the temps you may be able to match the thrust. If that didn't work you could then try Todd's method to open it up. Of course onece you cut or remove the material there's no going back.

Just saw your post above. What are you using for fuel, ACID?[X(]