I have a TS250/1, Supa5, and only have experience of a short ride on a ETZ250. Esentially I found that the TS250/1 pulls sensibly from around 3,000rpm whilst the ETZ needs around 3,500 to give the same sort of pull. The difference was nowhere near what I was expecting from reading the same sort of things you have. Definately recommed trying a TS before you make any decision.
Just so you know the cylinders are more different than just the porting. Stud size and spacing is different, crankcases are different with the start of the transfer ports being bigger in the ETZ engine. The cylinder is a different height due to the revised crankcase. It’s possible to fit a TS cylinder to an ETZ but requires a 10mm (I think) spacer under the cylinder scalloped out to merge the ETZ crancase transfer area to the TS cylinder, the cylinder studs of the correct size and spacing need fitting to the spacer. I thought I had a copy of the article I saw on this conversion but I can’t see it just now, if I find it I will let you know.
I think they modded the ETZ to match the characteristics of the then current Japanese bikes. Whilst Wilf wanted to stay with a cheap and cheerful commuter bike the Factory deceided to challenge the Japanese, so it got a peakier engine, oil pump, disk brake, 18 inch rear wheel and more current styling. Plus of course an increased price which definately didn’t please Wilf. Was this the correct decision? Answers on a postcard please.