Agree that retailers aren't usually very good with part numbers.
The original SS-***XP platinum units are all
So there were no units below 860W in the Platinum XP series. Therefore when retailers leave off the '2', when refering the Platinum-560 Fanless or Platinum-660 etc it is just a typographical (or some other kind of) error, but they are still talking about the new XP2 units.
The XP2 units have various improvements over the XP units, most noticeably smaller case size (190mm deep vs 160mm), but probably some other things under the hood. There are sometimes differences between the modular interfaces of difference generations of Seasonic units (eg KM and KM2 use different modular interfaces) - don't know if this is the case between XP and XP2.
I think the sales manager you spoke to was probably thinking of the X-560 when you were talking about a 520W 80 PLUS Gold model; they definitely don't/didn't have one.
Seasonic's 80 PLUS Gold units have different part numbers.
The original X Series (X-650, X-750): SS-***KM
Second generation (X-560, X-660, X-760, X-850): SS-***KM2
Third generation (X-650, X-750, X-850): SS-***KM3
X series (X-1050, X-1250): SS-****XM
IIRC the SS-***FL (eg SS-400FL, SS-460FL) were KM2 units also.
So the Seasonic sales manager you spoke to was incorrect about the '2' denoting efficiency.