Real performance boost with multiple partitions
I have never, and I mean never, even back to the "classic" Mac O/S days (going back to system 6) partitioned a drive into smaller chunks.

Performance gains were supposedly about different parts of a rotational drive being faster than others (EG quicker seek to sectors near the physical hub). There may also have been some O/S speed gains in some O/S if you keep parts on different partitions, though I am not entirely sure as it has been years.

That said partitioning a drive does have some benefits, mostly when it comes to backup as it can be easier to back up all of one drive allowing you to easily exclude those things you don't want or need. On the other hand most backup programs allow easy inclusion or exclusion of types of file based on location and multiple other criteria.

Personally I have never bothered with partitioning as the few times I have had to use it I found it limiting to what I wanted or needed to do on the computer. In other words I would occasionally run out of room on partition "A" with plenty left on the other partition(s).

There is nothing inherently wrong with partitioning a drive but there is no real pressing need (or requirement) to do that with O/S X. And if there is no need/requirement I go with the first principle of good engineering: "KISS" or Keep It Simple, Stupid ;-)

As for question two, I suppose it depends on the availability of available backup space. If you have enough room to back up everything, do so and reformat the drive as one partition, then restore what you need to the fresh O/S install drive and go from there.