<p>That is as comprehensive a response to “to prior-contact PA or not to prior-contact PA” question as it gets, and I try to pinpoint the main points (Although I know only of one field, EE, it seems reasonable to generalize them):
1 - It is to a large degree faculty-dependent, based on their attitude toward this matter
2 - Younger faculty members are more willing to do these prior-admission contacts
3 - Faculty in low ranked programs are more willing to do these prior-admission contacts
4 - “Thank you for your interest; please contact after admission” email is usually the best you can count on, except PA falls in category of 1-3.
5 - Even if a PA shows an out-context excited interest in your case the letter will certainly end with “Please Apply”
6 - Usually increase of chances is cannot be significant.</p>
<p>I would say it is “dumb” to expect anything substantial be gained by prior-contacting, because after all no matter at what time frame you contact the PAs they would rather hesitate to see if there better applicants out there that have not made a contact yet or have preferred following the sane rule of “apply first then contact”. In any case sending emails based on what I know is usually annoying and even if you have much to say you will end up an annoyance of sorts because they get a LOT of those and become insensitive to them (according to points 2&3 it is understandable). The sole purpose of this thing could be checking they have funding at hand or active projects going on which itself mostly is not clear to PAs themselves in say October prior to September of the next year! Of course you should find answers to a couple of questions on your own as your homework in order to see if that person is a PA by definition or not in the first place. Does s/he has at least 4-5 papers/year co/authored in last few years? Are those usual prof.-student papers or the prof. has been acting as sort of a consultant to other research groups? You can easily find out about these matters and no need to flood their inboxes.</p>