We fitted Millenium cylinders 300hrs ago to our 0-360 (Assume this is the same company which you refer) to. They were a bit more expensive than Lycomings but were produced with a better quality casting process and due to a redesign of the heads claimed better heat dissipation and cooler running than Lycomings. We've just had the engine back from being zero timed at Norvic as part of a shock load inspection (another story) and the only work they carried out on the Cylinders/heads was to replace the exhaust valve guides - more for the sake of it while it was in pieces than anything else. Don't know if you've spoken to Norvic, they might be worth having a chat to about zero timing your engine - they'll fit whatever cylinders you specify.