In general the more cores the better

Joke aside the performance limit for fast viewing is more likely to be the disk(s). If you have large images, transferring, say, 4 big files in parallel will challenge any hard drive, solid state or not.
For tasks like image export or other kinds of batch processing the disk r/w throughput will likely also be the first limit on a 6 cores machine.
The only part that really uses many cores to good effect is the number crunching part of the GeoClustering plugin, as well as some portions of some of the codecs.