In our department, the vast majority of our staff do 90% of their work in a web browser. A handful use a separate email program (Outlook, unfortunately), but most are in Gmail. They also occasionally use MS Office, but that software's crappy interface is equally bad regardless of platform. So I have a hard time believing switching them to Macs or Linux boxes would have any deleterious impact on their productivity, except perhaps in the very short term as they learn the OS' interface.
And, since we do have a number of users on Macs already... it's not as if our desktop admins would be dealing with an unfamiliar management setup.
Our group's desktop admins are considering making this switch right now, actually. It's got nothing to do with "increasing productivity", it comes down to per-unit costs plus positive experiences with those staffers who are already on Macs. The group has been long-time Dell customers on the Windows side, and what I'm hearing from them is that Dell's business laptops have gotten more expensive than the equivalent Macs. FWIW they have various reasons they've rejected Dell's competitors in the Windows space.