So... the only benefit for a starting Psyker making a Counter-Focus Power test is the +10 for the Psy Focus and a +2 (+3) max for his PR?
Not a great bonus, but still, better than the non-Psykers =P
Whether it's better than non-psykers is debatable, I would offer, because it comes with the chance of a Psychic Phenomenon (which, of course, includes the chance of a Perils of the Warp). If you had the choice, would you take +2% considering it comes with a 10% chance of a Psychic Phenomenon (and then a subsequent 25% chance of the Perils Table)? I would suggest that, perhaps if you were facing daemonic possession, you might accept the +2% - but in most cases receiving +2% for a 10% chance of a Psychic Phenomenon(& possible Perils) is dubious.
But on the subject of choice, it's interesting that the psyker gets to choose whether to "push" with this Resistance-opposed-WP-roll, but does not get a choice of whether to add their PR and therefore be subject to a Psychic Phenomenon.
Lastly, I would also point out that there's a notable disconnect in the fluff since this Resisting ability is described as "...psykers have far better mental defences than "normal" people....". I think even +10% would be a little bit of a stretch to describe it as "far better mental defenses" and that would require an extraordinarily powerful PR of 10. +1%-5% is certainly not "far better mental defenses" than normal folk. And your +4% is not going to feel better at all when you roll a doubles and then a 90, sending you to the Perils Table.