Max I have operated in such an environment for several years now without any problems whatsoever. I use all Dell equipment with various levels of Perc raid controllers in everything. You can't swap a 10krpm drive in to a 15krpm array, but the reverse has worked just fine on all of my Dell equipment. Not only that but I've used 15krpm drives that are much larger in size than the original 10k disk, and that also worked out very well. The only issue that I've experienced on some systems was that I had to upgrade the controller firmware to the latest version.
If HP and IBM are saying not to do this then perhaps their controllers can't handle it? Worth noting is that my 15krpm replacement drives have always been provided to me by Dell under warranty. The drives are usually iterations of the 10k counterparts (same manufacturer, etc). Perhaps a drive from a different manufacturer wouldn't behave as friendly?