I have been using Microsoft Office on Windows for a long time, from 97 to 2013, and occasionally I use it on a Mac computer as well. There has been previous questions about comparisons between Windows and Mac OS, and I think this is one comparison between the two platforms that might be easier to test in terms of usability (although there are obvious constraints due to it being a product originally designed for Windows). I would like to know if there are any studies done on this, or if there are some ideas about the key differences in terms of usability and benefits/downsides when used in each of the platforms.
I was a user researcher for Office:Mac 2008 and 2011, so I think I'm reasonably qualified to answer this question. That said, I did leave 3 years ago, so things might have changed since I left.
It's not accurate that Office was originally designed for Windows. Both PowerPoint and Excel were originally Mac-only applications. PowerPoint was originally developed by a company called ForeThought, which was acquired by Microsoft after they released their first version. The first Windows version came out in 1990, 3 years after it had been available for the Mac. Likewise, Excel was first released for the Mac in 1985, and its first Windows version came out in 1987. Even the Office suite itself was first released on the Mac in 1989, with the Windows version following in 1990.
Office for Windows has generally had a focus on enterprise users, whereas Office:Mac has generally had a focus on consumers. Likewise, Office for Windows optimizes its user experience for Windows users, and Office:Mac optimizes its experience for Mac users. There are, of course, users who use both platforms. Some use both platforms roughly equally, some prefer one platform and only use the other when circumstances require it.
A usability study comparing the two different versions of Office would need to have very specific research questions to have meaningful results. I wouldn't be comfortable making a generalization about whether one is more usable than the other, since they have different users with different expectations and different goals.