Here is the logic I used in my decision-making with regard to which Mac Pro model to buy.
Imports and most importantly preview rendering are most definitely multithreaded. I see no reason why Adobe would limit it to just 6 or 8 cores, and I would imagine if that limit existed, it's arbitrary and trivial to remove if Adobe so wanted to.
Running up extra cores takes a little time and effort, and so while the extra core would be useful in the editing workflow of Lightroom and Photoshop, core count is probably not as critical since the operations are relatively short-lived. By the time you get a thread going, and started to process a brush stroke, for example, most of it could be done with a single thread.
Also remember that input/output is part of the equation - and could be an important bounding factor on preview generation. You may have a dozen cores rendering previews, but they all have to go through a single I/O controller.
So, I went for the 6-core model because the clock rate of the processors is higher, and would give me a small advantage in editing (brushing local adjustments, spot removal, etc). I can wait a few extra minutes for preview rendering. Not to mention it saved me $3,000 - which I put toward more static RAM, which, as per my I/O argument above, is the shortest path to high performance.
Regardless of your choice, you're not likely to be disappointed. It's going to be a very nice machine to use.