neb:

mdf: I'd check with the manufacturer. I'm guessing here, but the composites I've used are both heavier and flex more than timber. 400 centers might be to address that?

They've said it's OK but will void the warranty, but then see @eracode's reply. I'm also going with the hollow-core rather than solid ones which are a third lighter, so hollow at 450 will still be less load than solid at 400.

The don't actually stock the solid boards anyway, so solid would be a special order. The sales person told be she has never sold any solid.

It's good they've confirmed it's OK at 450, but annoying that it still voids the warranty. If it's OK why does it void the warranty?

As an aside, the guide also says not to install near low e windows because of reflected heat. I queried this (since low e is very common, and will probably soon be standard practise) and no-one at Outdure NZ had heard of any problems caused by this. So I got it in writing from them that this wouldn't void my warranty.