Type 1, absolutely, it’s a credible, validated test. That doesn’t mean they WILL become symptomatic, though a P1/P1 horse is far more likely to be symptomatic, and with worse symptoms, than a n/P1 horse. If you have symptoms, it’s a good place to start.
But type 2? Zero credibility in any results meaning anything about becoming symptomatic. It may have value in looking at which type when there are symptoms which may help dial in the diet and exercise more appropriately.
Muscle biopsy only has value once there are symptoms, as its sole purpose is to look for changes in the muscle fibers, which are what’s causing the symptoms. You can’t use a biopsy as a predictor