For all you guys running Billy's, they now offer a rebuild service on the dampers in the UK but the great news is, they can rebuild them to any spec you like so if you want to increase the damping rates slightly to match the Eibach springs then this would be the perfect opportunity.
Prices start at £85 per damper, not cheap but you're essentially getting a new damper to the spec you want with Bilstein quality.

Call Bilstein on 0116 2898 345

Do you know if they can pull the same trick on other brands?

I doubt Bilstien would entertain anything other than their own dampers. It kind of goes against the grain and how would the valves etc fit?

AVO do refurbing of dampers/coilovers etc any brand so I've been told but don't know the price

I assume as the assembled dimensions of the unit have to confirm to a standard the innards can't vary hugely? I'd have thought as they can change valve size in their own product any other damper should be doable. But I get what you're saying about them picking.

Knight, thanks for the heads up.

I thought the Bilsteins were rumoured to have been developed to go with Eibach springs. Is this not the case then?

Not on the coupe they weren't, the Bilstein B12 kits come with Eibach springs so will be valved to match them, the B6 available for the coupe actually specify they should be used with standard length springs.

The bilstein rears especially on the coupe feel under damped for Eibach springs so improved valving would certainly help there. I'm sure they would work out a solution for you if you told them what springs you are running.