Honestly, if you're thinking about getting a wheel and separate H-a and OIII filters, you might be better served just going full monochrome. I mean - I do now use a 533 and an L-extreme and I accept the limitations of this setup and knew them full well before going in - and am happy with the choice I made (going OSC rather than getting all the different filters and equip for my SBIG).
I *don't* think it's a bad idea to use a wheel if, like you said, you want to change out the H-a and put in a broadband filter or something since doing so would help limit dust getting into the system and I might do the same, myself for my L-extreme and L-pro. I've also got an H-a filter that I'm not currently using and I guess I could drop it in there, too...
Have you tried imaging in just H-a with your 533? It might produce pretty clean images, but that's because in emission nebula the H-a signal is going to be really strong (relatively speaking). The OIII signal on the other hand is usually not, so when you're limiting your efficiency using a OSC and only grabbing blue/green light, I'd think the images would be really noisy compared to imaging OIII using a monochrome camera (?) I haven't tried this, though...
In the past I have done HOO stuff with monochrome CCD with H-a and OIII filters, and you usually have to get considerably more exposure time in the OIII channels and I just imagine this would be a LOT harder to do with a OSC...
