In a short answer- no- genetic testing doesn't help you manage pku.
By the time you get the testing done you will have figured out the levels of PHE needed to keep numbers in range . They will fluctuate with illness, growth spurrts, exercise , etc.
Once you learn the exact mutation you will not change what you are doing. What is useful is if you are trying to get pregnant and you are pku and you and your partner get tested you can find out how likely it is for the child to be pku and how severe it might be.
If you have to pay out of pocket i'd save my money and invest in more low pro foods!