Jacob has had the blood drawn, and we are currently waiting on the results of the test for him. (Like Foufoka, it is done for free here, in Ontario) My understanding is that once they know exactly what mutation he has, it will assist in knowing how he can be treated best. Yes, diet will be important, but it is our understanding, it will also help predict better which diet restrictions are best.
Originally we were told that Jacob's levels were low enough, that most likely he would only need monitoring until he was 5 years old, and then would be discharged from our clinic (assuming treatment guidelines in 5 years are what they are today) Low and behold, we are now on a diet, and monitoring is MUCH more frequent then originally anticipated. I understand it will take some of the surprises out of his/our future when it comes to PKU.
From there, once they know Jacob's strain, they also stated they can further test his biological mother and father (friend was donor in our case). This can give valuable information to our friends other children, who show no signs of the disorder, however, are likely carriers. It is also information that my partner can share with her siblings who may also be carriers.
This all being said, I am not sure it would be a step we would quickly jump to if the fees for the test came out of pocket, and not from government programs. I may have more information once we see and fully understand the results of the test, but that may be a few weeks or so to go. Thought I would share what I had at this time. Hope it helps.