Hi Jerry,
Firstly, I doubt very much the TXV was faulty. The TXV used is the same product (different setting) as that used by Toyota/Honda/BMW/Jag/etc etc and they very very rarely fail.
Basically, the A/C unit in the V8 really is a hacked up version of the superb unit designed and installed in the normal 75/ZT. I know the A/C unit very very well indeed, and can tell you that the OEM manufacturer of the system had no involvement in the V8 mods due to basically zero development budget from MG-ROVER. The unit starts live as the exact 75/ZT unit taken from the stores, and then striped, literally cut apart and then re-patched together.
If your system works to an acceptable level, then believe me it is just luck, and most of them will perform very poorly indeed.
No dealer would be able to improve the system performance, as it is awful to start with.
Kaine