Quick update. Got the M3 mounts. They seemed identical to the original's. Hard to say if they're stiffer, but I fitted them anyway whilst doing the cdv mod. Definitely an improvement in first to second, not huge but some. The originals were over 80 thou miles old, so could just be that!
On the CDV mod, I clamped the pipe with a proper pipe clamping tool, removed the valve and fixed the pipes back together. I didn't have time to bleed the clutch, so just tried it. Pedal was spongey as you'd expect! Left it a couple of days and it bled itself. Great mod, far better without it!