Don't you notice a real loss of suction
No, not at all. At least, not so much of a suction drop to hinder performance.
This all boils down to Dyson marketing, but since you're a Dyson fan, I won't go there.
I've never used a vacuum that, other than getting blocked, will not perform to an acceptable standard with a full bag. Even with a choked up bag, the suction has always been enough to clean a carpet. This is especially true of the Sebo X series as the filter runs up the side of the bag and is not sat underneath it, so the suction motor does not have a full bag of dust ontop of it.
I mean, how much suction do you think it takes to clean a carpet? You're only picking up dust and grit, not bricks. Dustmites don't wear seatbelts, ya know

.
So, whereever possible, I will cram the bag full.
18th century cottage? Be still my beating heart - sounds lovely! We're in a 3 story, 3 bedroom new build, so total opposite.