Hi! I believe it's called a "soft ceiling" when the test questions run out, but a maximum score hasn't been reached and other termination criteria haven't yet been met, for example if a rule to end after five misses in a row hasn't been triggered. It sounds like that might be the case with your son's 16s. If his scores are that high, I doubt that 1) you are going to get a substantially more accurate read on him with further testing, and 2) that it's going to help with early admissions to college.
That is, they'll already see he's wicked smart because of his scores and achievement. They won't be more likely to accept him into early enrollment, I'd think, just based on the score differences-- it'll be more about his maturity level and academic readiness.
Incidentally, it's my complete layperson's belief that you will run into fewer "soft ceilings" with the SB-5. But I may just be smoking crack.