Well all in all what you have is a pretty significant change. Shunt PSUs, local shunt regulation, another I/V stage, new board lay-out, etc: all are minor to major changes. I'm running the the BII and there the differences were more pronounced, as dual mono and Tridents were upgrades as the build progressed. IMO the improved local regulation is a bigger upgrade than the move to dual mono. But both are noticeable improvements.

So yes, dual mono is certainly yet another improvement. It's unfortunate the upgrade almost doubles the price of the DAC. But that's the downside of going high-end: the cost of further improvements rises substantially. If it is worth it is up to the builder.

Yes, but at least this is DIY. Not only can you build it the way you would like to, the upgrades cost less and hence the tresholds to an upgrade are considerably lower. Plus one could upgrade later on with almost no extra costs, which is significantly more difficult in the non-DIY sector.