More or less.
The memory size is configurable, with a minimum of like 2800, and a maximum of 6750mb. The 3390 default is where you get most of the speedups; bigger gives very small improvements beyond that. But if you're trying to fit more parallel processes into a fixed ram size, decreasing that number can sometimes be helpful.
As far as threads, only phase 1, which occupies about the first 1/3 of the plot time, is multithreaded at all. There is a decent speedup by going from 2 threads to 4, so if you have enough cores, it's recommended. Therefore, IMO the recommended number of threads for n parallel plots would be 4 * (1/3) * n + (2/3) * n = 2n. But if you want to run 2 threads only in phase 1, you'd only need (4/3)*n, which may let you run more in parallel. Which is better will depend on your exact hardware.