In a fit of sheep-like behaviour I ordered the same Bagster seat from chomeburners in the Netherlands. The cheapest version was black and silver (non-bultex foam, or Bagster Ready as they term it). I've been reading this thread with some interest as a) it doesn't seem any more comfortable than standard and b) it is a poor fit.
After reading @coachluciano suggestion I swapped the rubbers and it seems to fit a bit better.
Here are some photos, firstly the front set rubbers are in the left hand photo, Yamaha OEM on top, Bagster below. There is virtually no height difference, but the Yamaha versions are wider and may help with stability. They're awkward to fit into the Bagster seat so I've left these alone.
The rear sets are in the right photo, again Yamaha on top, Bagster below. You can see there is a considerable height difference so I've swapped to the Yamaha rubbers and the rear seat sits a lot lower now.
On reflection, unsure at this stage whether I've simply wasted £200.
[Edit: on mobile, front rubbers are top photo, rear rubbers are bottom photo]