Hi there,
I've seen nearly the same problem.
with sieve size 16384000 the smallest WPS i can reach is 211.
(Dual Prozessor Xenon E5 with HT enabled, 32 Logical Prozessors!)
With the new updates you should no longer aim for WPS 20. Just use the default sieve and increase it if you wish. There is also an auto-tune option enabled by default which will run for like 4 hours