I have used two "W.38.800.8.MC" from two different series and I think they are quite weak speakers, so I don't know if the "W-38-1200-8-MC" will be much better.
Currently, I have loaded 15TBX100 and the difference is big.
Both the STX W.38.1200.8.MC and W.38.800.8.MC look good on paper, except one thing, the weight of the drivers doesn't correspond to such T/S-parameters: Normally you wouldn't use the physical weight of a driver to base it's performance on, but since all these drivers have ferrite magnets and from the looks of the photo's also have aluminium baskets, you would expect better overall performance with an increasing physical weight.
The 1508-8 SPS BWX for example, weighs 7.7 kg but it has a high-ish Qes (less magnet needed) and small Xmax (4.7 mm again less magnet needed), so those numbers add up.
The Kappa Pro 15LF, weighs 10 kg, has a medium Qes and Xmax, and a 3" voice coil instead of a 4" voice coil like the Black Widow has. Again those numbers add up.
The STX W.38.1200.8.MC, weighs 7.5 kg, (claims a large voice coil and Xmax, medium Qes) and the T/S-parameters suggest it would would be close in performance to the 15TBX100, however the 15TBX100 (also large voice coil and Xmax, medium Qes) weighs 12.3 kg.
So something is off, as I'm rather safe than sorry, I would exclude the STX drivers from the list.
1) Kappa Pro 15LF
2) Black Widow 1508-8 sps (or the Kappa 15LF, 9 kg but a steel chassis)