In addition to the above, a longer wheelbase can smooth out the ride and ease hill climbing especially on steep grades.
But in addition to those downsides there is the extra weight of the arm. They aren't ubiquitous in part because they cost a good deal of money; in addition to the arm you need a new bearing carrier on the 660 and bearings plus a chain guide or slider or perhaps both. You'll need to re valve and or re spring the rear shock to handle the extra leverage. If you go too long you won't have enough weight over the rear tires and traction will suffer as will your drag strip times. Regardless you will need to rethink your tire and gearing strategy again due to weight distribution.
For the 660 the +4 seems to be the most popular middle ground for dunning while still being good in the trails. If you spend most of your time in the trails I would suggest a +2, if you are racing at the strip, up a sand or dirt hill, or just on some flat ground somewhere then it depends a lot on the terrain and how much motor you have.