Good questions MOTF! My personal experience with IQ testing only goes up through age 10, but I am "well read". That said, for DYS purposes you absolutely want either the WISC-IV or the SB-5, and given your son's "weaker" (relative) math, my guess is he'd be better off with the WISC-IV.
Hmmm...interestingly, he actually scored higher in math on the PLUS (CTY when he was in 5th grade) than in English. However, that may have been a fluke?

All scores were quite high. In 2nd, he scored 99th percentile on the CoGat in English and 97th in Math. That seemed like a more accurate assessment. Given his one low subtest score in Math on the ACT, would he do better on an IQ exam (either)if he were to be given exposure to the math information that it appears he simply hasn't been exposed to in the past?
But...here is where his challenge lies. To get 19's, he could very well have to answer most, if not all questions in some areas. This hurts him, because it gives him no chance for "stupid mistakes". The younger the child, the more "stupid mistake margin" exists.
Hmmm...not sure if we want to give up our "stupid mistake" margin!

Unfortunately, DS doesn't have the more common "perfectionist" gene that tends to come with the gifted. DD has it, but not DS. A couple of years ago, his teacher, and award winning Math Olympiad coach, commented that DS may be the "smartest" kid in all areas in his class (self-contained gifted classroom) - but that he makes silly mistakes - like forgetting to carry. Mr. Teacher told us recently that DS would be the perfect part of a math team - because DS could figure out what needs to be done conceptually, and then allow the other students to do the actual work.

Not exactly sure what to do about that!!
Hopefully someone with 10+ testing experience can chime in with first hand thoughts.
I'd love to hear from them too!

The SB-5 goes up through adulthood, and therefore might have a higher relative ceiling, but again, some believe it favors spatial/math kids over the more verbal ones, although this is far from proven, and some people have had unexpected results with one test over another.
I would love to hear more people's take on this. DS doesn't love math, but still does well overall. Would his one low subtest score on the ACT (remembering the other two much higher scores) be enough to keep away from this test? Of course, you are right in that he much prefers and tends to test even higher in Language Arts subjects.

Thanks for all of your insight and advice! I truly appreciate it!! And, the more insight and advice you (or anyone else has/)have, the more I'd be willing to listen!