I agree, I've used this setup live for years, and it mostly works, except for some occasional CPU spikes.
HOWEVER, I'm in the process of changing over. AcousticSamples has a wonderful new B5 organ that's even better than B4-ii. Unfortunately, their Leslie emulation isn't great (they are working on it), BUT Melda has an amazing Leslie sim, MVintageRotary, that's on sale for only $27! This combination has turned out to be the proverbial holy grail for me, and absolutely destroys B4-ii and VB3. I haven't finished integrating it into my workflow yet, but I HIGHLY recommend for anyone starting out.
Here's some info. AcousticSamples makes amazing UVI Workstation based instruments, but for their first version of B5, they simply used the pre-packaged UVI rotary sim, which just sounds kinda weak. When I've questioned them about it they were very frank about the fact that they are working on a huge upgrade, my guess is they are developing their own Leslie sim, and judging by the quality of their other instruments it'll probably be amazing. So keep checking back. In the meantime, Melda's sim is cheap and really really good.
One more thing to note, a single instance of B5 uses a scary 500MB of RAM, however, UVI is pretty smart, and all subsequent instances in a project use the same samples and don't take up any more RAM. That's okay with me, anyway.
EDIT: Sorry, but IMO, Apple's Organ is far inferior to any of these options. I've spent hours trying to get it up to the harmonic juiciness of even B4-ii, and it falls way short. But by comparison, B5/Melda is about the same level of upgrade to B4-ii that B4-ii is over Apple's organ. Seriously... I'd even take NIs Vintage Organs (which is an absolute disgrace in of itself) over the MainStage organ. Apple makes amazing software interfaces, but I've never encountered a VI plugin that even a low-end third party didn't absolutely destroy. Some of their effects are good, though.