Duckst, an interesting question.

Operator error may show differently with differing rod action. I'd even hesitate to say error, as it may just be not adapting the stroke to the different rod actions.

The backcast usually develops less power than the forward cast. This is due to body design and practice. (It does usually does develop a quicker stop, however - just for these reasons.) And if you use a wrist snap for the quickness at the SUAS, i.e. power snap, it especially generates less speed/power on the back stroke.

The SLT, being a bit slower, perhaps, works best with a slightly longer, smoother stroke. The fast, quick acceleration isn't as necessary to get the good backcast as with the Z.

My guess is with the faster Z, a quicker, shorter stroke is needed - and you're getting it on the forward but not on the back. And the faster, quicker forward is not working as well with the SLT.