Hi Ben. Given that the IZAs and IPAs have a comparable wheel base to the VGA made by Farish, I can say that I have never had any drag-related running issues with those. I can't, however, comment on the VGAs capabilities in terms of small radius curves, as I don't have anything that tight on my layout, so, unfortunately, I'm unable to provide any sort of comparison for you.
With regards to the running faults that you mention with the early IZAs, and also the small number of ground-down axle pin points fitted to the IWAs, it does appear that I may have just been very unlucky, probably, by the sounds of it, just managing to receive a high proportion of that very small number in the ones that I purchased. This, however, doesn't take away from the fact that I have had to adjust factory fitted detail, and to also swap out manufacturer wheelsets (Kindly supplied by yourselves to rectify the issue), to allow for free running of items sold as Ready to Run models.
As for the Farish BRA/BYA, I have three or four where the wheel flanges (factory fitted axles) make contact with the chassis outriggers, and as a result, these ribs have had to be cut away, fortunately, in this instance, not detracting from the model in any way.
I must admit that the issues with the IPAs may not be down to the wagon itself, but probably more to do with my choice of traction. Strangely, if not ironically, it's only the Revolution Class 92 that has issues pulling a few sets. Perhaps these were left in the oven for too long?
Apologies for taking this thread off on an RT tangent. It was only my intention to illustrate that Dapol are not the only manufacturer to be infallible, and whilst saying that, I continue to support all that were mentioned.
Cheers, Paul.