It's OK, stand down. I need to eat humble pie for focussing too much on the problem without being more discerning of settings!
I realised now that I didn't have Cycle Exact Full/DMA turned on, which as soon as I did has undone the speed-up I was getting. Indeed now I've finally had the chance to get on my real Amiga and run the same test, I get none of the speed boost.
Curious what the difference is in the emulation there that would give such different speed results if vpos < $1f or >= $1f, without cycle exact.
Either way, chock this one up to me being a dunce.
