I’ve done a few large migrations.
There is a bulk importer that is much faster, but is not as full-featured. I think that you can probably use it in conjunction with the traditional importer, but haven’t tried it yet. The regular importer will probably take a week or two for the first run. Subsequent runs are faster since they skip already-imported data.
301 redirects are included in the standard importer, though most everyone reports a drop-off in incoming Google traffic after a migration. There are a number of topics about that here.
Category-level moderation isn’t on the public road map that I can see. See Category-Specific Moderators, phase 1 RFC for a work-around.
The uber-long topics should probably be converted to categories with topics split out by number of posts and/or time-frame. Whether to do that by hand before the import or modify the importer to do it is a question to consider.
If you have migrated your forum from other fora in the past then that can complicate things, as it’s likely that the previous imports left things in ways that were non-standard. The biggest problem that I’ve seen is wanky formatting.
For big imports I’ve done mini-imports of just recent topics so that you can get an idea of how things will look without a week or more of compute time to import all of the data.
Discourse Migration – Literate Computing, LLC contains my notes on migration. It’s designed largely for potential customers, but includes information that anyone can consider (much of which I’ve already covered here).