From the PSE site.
"The stamp is examined for creases, thins, color fading, toning spots/stains, tears and reperforation or other alterations."
The stain is considered a fault.
"As our experience with grading has evolved, it turns out that such faults affect the appeal and marketability of an otherwise 98 or 100 grade stamp
more than that of an otherwise faulty 30 or 50 grade stamp."
So yes, it is an Extra fine stamp with excellent centering, but for grading purposes the stain would hurt it a lot. I have no clue as to what it might grade out at. IMHO grading should be left for Stamps pre 1900.
Here is the link to the PSE grading criteria.
http://www.psestamp.com/pdf/2009_Gr...e_092009.pdf