Good that you found a couple of the RB APOs. those indeed are not common. I like the RZ a lot better than the RB so it's an easy choice for me. Although the RB APOs are interchangable with the RZ, you have that extra register distance in there, which is a bit of a pain. Plus it would be easier/cheaper to find the RZ APOs so if you decide on the RZ as a platform, I'd sell the RB APOs and get the RZ versions.
I have the RZ 140 M/LA macro, which is the latest version, I think corresponds to the K/L RB version. It's a great lens, not just for macro but for everything. Particularly nice for portraits. Definitely worth having if you are using stuff in that focal length. If you switch to the RZ platform, but 110/2.8 is a superb lens, as is the 50 ULD (both unfortunately unavailable on the RB platform)
re: Sports shooting: I generally shoot the RZ handheld, with the L-grip, motor drive, and AEII prism finder. It is a little hefty but manageable. I have even shot the APO 500 this way at an air show (minus the L-grip since it gets in the way of the lens bracket). Depending on the sports, pre-focusing definitely helps. Most sports shooting even with 35mm AF works better this way in a lot of situations (pre-focus). One will likely know where the action is going to be, and can usually set up to catch it when it happens. some sports are easier for this than others of course. Although I have the lens bracket for the 350, I rarely use it, and mostly go without it, but just support the lens by hand when necessary. This allows continued use of the L-grip which otherwise interferes with use of the lens bracket. (e.g. it's difficult to use the APO lens brackets at the same time as the L-grip).
the 1/400 shutter speed can be a little limiting but depending on the sport and which way the action is moving, it may be a non-issue. Even if 1/400 is a little slow, sometimes creative panning can really add to the shot anyway, from a visual/artistic point of view.
-Ed
