I don't know of any reason you'd use a Pro-chassis bulkhead over the AXYS, but I don't think there's much to be gained anymore from taking existing chassis and rebuilding them. Back in the EDGE and IQ days (and even more so before), there was a lot of weight to take off. Now, not so much: you could take a nearly 500lb EDGE and easily remove 50lb; not sure there's even half that much you can take off an AXYS using the same measures and level of spending. That's mostly based on what I've read of lightweight AXYS builds, don't own one myself.
My thought is that the movement we've seen in the last few years is in the wrong direction to some degree. Polaris threw down the gauntlet by knocking a ton of weight off with the Pro chassis, and continued with the belt drive, but there has been no movement on weight since then. In fact, the 850 has added a few pounds, the Matryx chassis loses no weight, and the developments I'm hearing rumblings about (turbo 850 and a 4T option) will also add weight. Surely there is another way. Unfortunately, while I believe the technology is there, and the gains could be huge, it's not going to be cheap. What could bring a tremendous shift would be a carbon fiber bulkhead and maybe even tunnel. I don't know enough about carbon fiber to get into the technical aspects, but I do know that it would take a major effort – it's not just a matter of imitating existing designs using carbon fiber instead of aluminum. Who knows how much that would cost, but it would be a lot more than existing models. Costs are coming down though, and eventually a full CF sled should be attainable. If so, my vision would be a <350lb sled. That kind of weight would mean no need for more than a 155 track, and a NA 800 (or maybe even a slightly smaller, higher-revving motor) would be enough power for most riders. There are a few wildcards: maybe a 16" track, monorail or narrowed twin rail, maybe a mono