I used an FC900 on a Mac for a while, and it was fine. Personally, I think the easier way to swap the Alt and Windows keys is to just do it in the keyboard preference pane. That even allows you to do it per keyboard, so if you use a different one on occasion, like an Apple keyboard, your change only applies to the Leopold.
I'd also recommend that you check out the Varmilo VA108Mac. It has Mac and Windows modes that swap the keys for you, and has all of the proper Mac keycaps legends. The only downside is that it only comes in all white, and the case has a slightly weird woodgrain finish. I took a look at their keyboard customizer, and while I'd love to order something custom from them, shipping is over $100 to the US, which is a total dealbreaker. They won't sell keycaps separately, either. I'd kill for a set of their Mac keycaps in something other than white.