I've got two apple xserve servers, one old dual G4 with regular 4 ATA drives, and the new one with 3 SATA drives. I bought them both with smallest/cheapest drives, and then replaced the drives with off the shelf bigger drives.
Absolutely no difference in reliability/performance.
I can say that cause older Xserve drives were still running good for 5 years when I replaced them with bigger drives, and new xserve is still running good.
Drives modules is no different every manufacturer has their own custom drive trays, which fit their servers only, Apple is doing nothing different here. Even our Mac's have had drives and other hardware replaced with off the shelf parts, and they work just as good, drives ironically were replaced from some Mac's cause original Apple ones failed.