I have a number of users running Mac OS X 10.9.5 with the new Outlook 15, but they're stuck at v15.6. Requiring a still quite new OS for Enterprise software (where OS upgrades are handled pretty conservatively) is very problematic.
I'm still testing Yosemite & while I've been reasonably pleased with it, it's still not completely problem-free, so I would prefer not to upgrade all my users, particularly those with older machines (or even newer ones with less RAM).
I am running v15.8.2 (15.9 was prone to crashes for me), but I keep getting complaints about performance problems with users stuck on 15.6 (esp. when trying to search their mailboxes). One has a larger mailbox (24.3 GB), but the other's is only 8.21 GB. Both are on laptops with 8 GB of RAM. I can't tell if I haven't seen these problems (I have no problem with searches) because I have a newer, faster machine with more RAM, or because I am able to run later versions of Outlook.
Anyway, it seems really problematic to switch the requirements for Outlook 15 from 10.9.4 to 10.10 midstream.
Tim King