Yes and no!
I bought one, and it is bloody good indeed. I really don't think they're worth �400 though!
However, not to worry, you can get them from Hornig in Germany; they sell all the TT stuff and will ship to the UK at German TT shop prices (unlike TT Germany who will not send parts to the UK, so you have to buy from the UK shop at massively inflated prices).
I got mine for a still expensive �300 (depends on exchange rate a bit) - brand new, TT box and instructions, exactly as you'd buy from the uk for �400. Buying this way also works out cheaper than the Kahedo seat (same as the TT seat except the branding) from OTR too, and you get to keep your old seat.
FYI the high seat is not, as they say, 4cm higher than the standard seat; it's 4cm higher than their low seat. High is +2cm and low is -2cm.
http://www.motorcycle-luggage-shop.com/Touratech/51817__Yamaha_XT_660_Z_Seats.html