There is a member of a forum I frequent - CNCZone - that speaks highly of G-Penny in general for their ability to machine at lower speeds than most other spindles available. He has discussed differences internally that account for this, irrespective of 2 vs 4 pole.
I highly recommend asking your question there as well, as I am sure he will answer.
He goes by @joeaverage, but @mactec54 also likes to join most discussions - the two do not always agree, and you will learn a bit about them through the banter 
here is one discussion you may find interesting, although related, not exactly what you are asking…