I have a TM hi capa which has been modified by Mike Cripps including cnc slide and all the usual extras. I love it to bits but for around half the year you will be lucky to empty a mag, let alone lock back. For all the money spent, I would actually say my new standard TM hk45 shoots virtually as well, rate of fire is amazing,gas efficiency is great and I wouldn't want to modify it until something breaks.
The only time the hk45 feels 'plasticky' is when you initially rack it, after that you will forget the plastic. If you are after a great skirmish pistol get the hk45 (or utterly brilliant TM Desert Eagle which my left handed friend uses), leave the TM modification game until you can justify a fun toy that's not quite so good on the field half the year.
I do have a WE glock which is modified for the full auto thrills, but it comes nowhere close to the hk45 really.