Well it appears I have a timely response to the above given that I have just driven for around an hour, often deliberately with both windows down (for testing purposes not to sound like a gangster).
They HAVE fixed this issue. No idea how. With windows down my sub no longer completely cr@ps itself. In fact to a certain degree of volume it now sounds and feels like a much bigger sub - with some house / ibiza tunes on, my rear bulkead glass / tailgate was visibly shaking with it.
I think they have also brought the inbuilt tweeters in the front door speakers into play more too. Certainly no need to put additional ones in the pillars now.
So all in all it is quite a revelation!! I'm also thinking that the system is now 'just' loud enough at max volume. I think there is a certain degree of trickery as the volume bar goes all the way to the end of the scale before it is really properly loud and doesn't seem to really get going until 2/3rds of the way up. This update certainly feels like the system is being driven harder and as my car has 30,000 miles on so the KEFS are properly run in, I no longer have a desire/need to change the system itself.
Pretty happy this end.
Cheers all.