Actually, Xserves will run with commodity drives. As the server drives filled up to capacity, I always tried to find an replacement match against the list of drives that Apple used in the Xserves but failing that I'd match a newer version. Some were enterprise and some were regular off-the-shelf units. A pair of 400 GB Seagate ST3400832AS that I installed into an Xserve almost 3 years ago are still running without a hitch.
As far as tweaking firmware goes, a lot of that can be done with the drive manufactures disk utilities. The only issues I ever had was that I had to slow down some of the newer drives from 3.0 Gb SATA to 1.5 GB SATA to use in the older Xserves. The "drive specific" rubber grommets did make me laugh - if vibration is a concern at that scale, then just use 'Sorbothane' for all of the grommets.
All that being said, I regularly replace clients drives around the 3 year mark - mainly, because I don't have a lot of faith in the longevity of any drives that are currently manufactured.
In seeking the unattainable, simplicity only gets in the way. -- Epigrams in Programming, ACM SIGPLAN Sept. 1982