The problem is that segmenting Products currently results in the check just looking for the value to exist (either one of the products equals the specified value, or one of the products contains the value), but all products are returned... And there is no concept of "number of products" (unless you happen to be sending a custom dimension with such a value).
That said... I wonder if you could do something like:
HIT [
Product SKU equals Professional Membership
AND
EXCLUDE HIT [
Product SKU does not equal Professional Membership
]
]
Basically, find all hits that have "Professional Membership" and exclude all the hits that have anything other than "Professional Membership"
This, in theory, should work... getting rid of the hits that don't have "Professional Membership" at all, as well as getting rid of the hits with other products...