So I finally decided on the FMX 178LR 91 board. A few thoughts about this outcome

My old FMX has given me a great couple of years service but it has certainly seen better days. All reasonable offers considered. $$$

Most of our foiling races are in lighter winds with the switch to 7.7 and fin at around 20 knots. My 7.0 foil sail still feels comfortable on 91 and since the 85 isn't available for 6-9 months I'll make the most of the familiar size board. 6.0m foil race sails are also as rare as 81cm boards!

Looking at the 85 Veloce (free ride) board over the top of the 91, 2cm either side wasn't really enough of a step down to worry. For a small foil board I'd probably want to go under 80cm. If an 85 FMX or equivalent was available I'd probably have chosen this size instead. But the 81 starts to get a bit to small for lighter wind days. I assume the smaller size would be fine in the air with say an 8.0 or even 9.0, but 140L vs 180L makes takeoff that little bit harder.

30+ knot speed runs will have the advantage of new low drag nose shape so I'm sure they will be easier.

It's cheaper than a race car!

91 was in stock!
thanks Remi at theWindSurfingShed.