It is an interesting question for sure. The Vapor has plenty of volume and a wide tail. It is super-stable for 26" wide, and feels an inch or two wider in terms of stability. There is no washing of water on the deck even in rough stuff. So why would the FX need more volume despite being an inch narrower and having a slightly more pulled-in nose? Clearly it is bringing some definite benefits otherwise SIC wouldn't have gone to all this trouble. As a rough waters paddler I know that there can be a cost of high volume in terms of stability and handling in chop. You don't want to be standing far above the waterline in tricky conditions if you can avoid it. So, the volume of the FX is a bit of a puzzle to me, and I'm waiting to find out why it is there. Mark Raaphorst knows his stuff, so there must be a reason. My SIC X14 is only 6 litres more volume than the FX and it is 28.5" wide and is often quoted as a good flat water board for big guys who like stability. So why would a board 3.5" narrower need the same volume? I presume that a lot of that extra volume comes from having a flat deck. But the Bark Downwinder has a flat deck, and is 28" wide, but still has only 275 litres of volume compared with (apparently) the FX at 290 and 25" wide.
But... taking too much heed of manufacturers' volume specs can lead to madness, methinks. Maybe the 290 figure is yet another of these "lost in translation" moments that has often happened over the years in brands' marketing literature. So until the long wait is over and you guys have your new FXs, I'm going to reserve judgement.
I wonder how long we can keep talking about this FX board without any information to go on! This board sure had better be something special when it finally arrives. It speaks volumes (apologies for the pun) for SIC brand loyalty that you guys will put up with this uncertainty and crazy-late delivery. That really is something that SIC should think carefully about. They owe you, IMO. The board should be coming with a handwritten apology from MR himself, and free t-shirt (at least). And it would be useful if that note could explain why the FX has (or is quoted as having) such high volume!