Soren, with all due respect, you don't know what you are talking about. Loading cartridges to higher pressure will not increase rof to any noticeable degree and "underloading" as you say, will adversely effect rof to any noticeable degree but will effect reliability
9mm sub-guns like the Cobray M11 (1,100rpm) do not get noticeably faster with +P+ ammo and do not get noticeable slower with plinking ammo that doesn't have the same oomph. And yes, I can state this from personal experience. The M11 is blowback operated BTW, where you would expect a difference in rof if indeed your premise was correct.
You said, "~400 rpm is probably the lowest RoF you can reach with the MP-44 before it wont cycle the rounds anymore."
Just where, exactly, do you pull these little pearls of technical opinion from? Do you have some data somewhere that indicates this? Or some data about the MP44 firing safely and reliably up to 750rpm? Is this from personal experience? Do tell.
"The MP-44 can be fired safely and reliably up to around 750 rpm I would suppose, however achieving this would involve using lighter metals for the reloading mechanism."
More pearls. Using alloys or other lighter metals to increase the rof? Hmmm. Interesting that no other guns use this principle to increase firing rate. Hey, I have an idea, why not just use a lighter spring or modify the gas port Einstein?
"this is obviously either because of a difference in powder type or the size of the charge - which would explain the slightly lower RoF."
Or, and this is a radical idea, it may be a 500 or so rpm gun which is pretty much what the video showed.