Just to report back on my findings and finish off the thread. The distortion I was seeing was due to the view from the mirror being through a lot of the near horizontal rear window glass, the line of sight is at a very narrow angle to the window rather than at almost right angles with most cars. The mirror itself was perfectly OK.
I fitted the 156 mirror but unfortunately the mirror stalk is longer than the GTV so the mirror was sited lower down and gave even less view below the boot lid lip than before - changed the mirror stalk for the GTV one and that problem was gone.
The 156 mirror unit is 53mm wider & 5mm deeper than the GTV one, I found there to be no noticeable blocking of the forward view. The mirror glass itself is 51mm wider & 3mm deeper than the GTV one, the extra mirror depth did not improve the rear vision but the extra width gave noticeably wider rear vision. Any improvement of the poor GTV rear vision is welcome.
The 156 mirror does not look over large, it looks just fine, actually I think it looks better. The passenger sun visor clears it by about ¼", the driver sun visor by a lot more.