I installed the Contis and did a test ride (I'm going to run a few samples obviously - one ride is insufficient). Here are a few practical notes:
1) The bead track is very narrow - 13mm, while the brake track is very wide - 25mm+ - but the 25mm and 23mm GP4000s fit fine. The 25s are a little fatter than 25mm, and so don't line up perfectly with the rim width, but it's pretty close, so I'm guessing the aero penalty is small. The 23s seem to fit pretty well, though.
2) The Mavic CXR tires are TINY - very low apparent air volume. For the heavier rider, I suspect that this is a disadvantage.
3) The gap-filling blades fit fine, and are "soft" - I doubt they will cause any excess wear on non-Mavic tires, despite the marketing claims, unless maybe some grit gets in there.

I checked / adjusted the bearings and there are no obvious issues, and there is no brake drag.

Ride-wise, the GP4000s *feel* like they roll better - the CXR tires are comparatively plush and have plenty of grip, but they *feel* slow (they remind me somewhat of the old Schwalbe ZXs - I haven't ridden those for a few years though). I suspect this is partially a weight issue - for the same pressure, they deform more. Again, though - I'm 95kg running a 25mm tire that's actually more like 27mm at 120psi in the back, and a "bigger" 23mm tire in the front.

There is enough noise in a one-ride sample to avoid drawing tire-based conclusions just yet, although it does seem like - despite good rim stiffness - the total wheel stiffness is not super-high (spoke count). I have only butt-dyno data for this - although the evidence appears to be that up to 350w this is not that relevant.

I might - although it's a pain - try running my old wheels with the CXR tires. That would be interesting...

Anyway, thanks for the suggestions - I should have some more useful information later this week.