Taking a closer look at the saves, the Mac version appears to have 2 extra global variables. I'm working on getting a copy of the Mac main.scm to find out what the difference is. If that is the only difference in the save structure, a converter may be possible. Although if it's like San Andreas, some other things in the save file might need to be converted as well, such as thread timers.
I got a copy of the Mac script and it is indeed different from the Windows scripts. Many of the thread names are different and there's some new code inserted in to it.
Using the Mac script on Windows I am able to use NSanityHD's save on both the Retail and Steam versions.
I'm still looking to see if conversion is possible.
I updated GTASnP.com to reflect the info. I'll work on conversion tomorrow if I have the time.