While I'm sat here waiting for my drive to update I'm just wondering whether you can use your fancy incremental magic when updating the Rescue Media for a faster update/copy?

A typical Rescue Media build is 300-600 MB, depending on WinPE/RE version and how many drivers need to be added to your build.  That should only take maybe 10-20 seconds to copy onto even a garden variety flash drive.  If you're referring to the time required to actually update the Rescue Media file set when that's required prior to copying the files to your flash drive, that part involves using Microsoft tools (i.e. not Macrium tools) to build the boot.wim file that ends up on your flash drive, and that process does take a while even when you're using the tools directly in other contexts.  But even if doing that incrementally were possible, the time required to mount the existing WIM file, compare its contents with the new files to see what needs to be updated, and then commit those changes to the existing WIM file might end up taking more time than just building a new WIM from scratch.  But fyi, if you're using Reflect V7 (despite having posted this in the V6 area of the forum), and using WinRE, AND you don't need WiFi support in your Rescue Media, then unchecking that option might save you a lot of time.  On my PC, a Rescue Media build that includes WiFi support takes about 2 minutes longer to create than an otherwise identical build that doesn't.  And if you ARE using Reflect V6, then fyi V6 support ended more than a year ago.