Hello Julian,
I can't give you a definite answer off-hand, but I know that in the past Apple have under-stated the maximum memory their computers can support, based on the largest available size of memory boards at the time the computer was launched. E.g. my 2008 MacBook Pro was specified by Apple at the time as supporting a maximum of 4 GB of memory, whereas when larger memory boards became available it turns out it's actually capable of using up to 6 GB.
I believe the same has been true with Mac Pros as well, although I don't know if that applies to your specific model.
If Crucial give a guarantee of compatibility then you might not have much to lose by ordering it and seeing if it works. I think there's a very good chance it will if they say so.