Slight Correction mike:
The GTX familia did not have the B6T in it... It had the BPT which is a 1.8T, just like the BPD which is also 1.8T but with stronger internals and a larger turbo with more boost. If I was to do a swap on a MX5 it would be with a BPT, as the halfcuts cost around 2 grand (The MX5 has the BP engine. I have done this conversion before, to my old astina. That thing hammered). The BPD halfcuts are around double that price, are hard to find too. That being said, the BPD is much stronger, but if you keep the BPT stock you will find it hard to break it. I flogged the tits off mine and it was fine. The BPT has 140KW at the fly and hits boost by 2k.
Also, the B8/BP/BPT/BPD are EFI whereas the B6/B6T engine is a carby.