Hi Alan,
Interesting and valid post!
I upgraded to a t/m a few months ago after my last portable (500psi triple vac). The reason for upgrading for me was speed not quality of work.
As well as speed of work having instant HOT water as most will agree is a real advantage to the cleaning process.
At 500psi with my old porty I would be emptying and re-filling after every room, not any more!
I still achieve great results (in my humble opinion) but in less time.
I still use between 250-500psi same as last porty.
I still pre-vac most carpets removing heavy dust and major debris.
I still agitate using envirodri on really trashed carpets or heavily solied areas.
I do not achieve better results, I do however achieve the same results faster and with better drying times.
The reason for the faster work times are because I'm no longer loading and un-loading a porty out of my van for every job, not looking for power supplies, not filling with hopefully hot water (if cold waiting to heat up) and not measuring out and mixing relevant rinse agent with every bucket into machine and not emptying and re-filling machine after every room cleaned.
Now I pre-vac (when needed), prespray 1,2 or 3 areas (depending on size) pull hoses from van, start up t/m, prespray next areas to be cleaned and start extracting on first areas sprayed, working back towards van all the time, pre-spraying areas 10-15 mins before expected extraction time.
Sounds harsh but using a t/m is a lot faster, I am doing twice as much work in the same time with my new machine as before with a top end porty. Not necessarily better results just faster working and drying times using the same methods.
*Must add - I did an equally good quality job with every porty I've owned, just took longer the lower powered machine used
Hope this helps.
All the best,
Jason.