Thanks for your feedback.
Regarding the W-irons, with the ballast wagon we were constrained by the fact that we did not have direct control over the manufacturing process until a very late stage (documented elsewhere). When you contacted us in relation to the W-irons on the ballast, we listened and made adjustments to the bubble chassis. However, we were again constrained by the fact the same chassis design was required for the bubble, as well as minimum manufacturing tolerances. If we had made the W-irons any thinner, the structural integrity would have been compromised and they likely would have warped while the plastic cooled after exiting the mould. We used the same method to accommodate re-gauging of our Accurascale 24.5t hoppers and customers report being able to drop-in EM and P4 wheelsets with little or no adjustment required - the difference being we had complete control of the design process for this model from the outset.
Regarding the wheelsets, the recognised tolerances for 21mm gauge are governed by P4 standards, which means that RTR RP25.100 OO gauge wheelsets are incompatible and require replacement in any case. As you say, the the Taras, etc. have wider axles and it should be possible to fit P4 wheels directly to them.