captain optimism, I think that's a good idea to have the testing, if only because gaps in ability are really annoying to gifted kids. My son (11 years) also has a sizable gap between language arts and math. He was just diagnosed with ASD (being specific as to type is no longer the norm, here, but he'd definitely fall more into the Asperger's end of things). Even without any other challenges, even if he was very "neurotypical", I think the gap would still have been frustrating. Problems explaining your work behind your answers in math, spelling out word problems correctly and reading word problems properly can make the "fun" class (in this case math) less fun. Also, low marks in language arts will feel even lower to a child capable of much higher grades in math and science. The more help my son got for his reading, the less anxious freaking out we got, and a way better attitude about school.
I also suggest finding and ally amongst the teachers who enjoys "mathiness" (I love that word you made!), it doesn't have to be the classroom teacher. DS1 was very lucky to have a teacher (actually the reading recovery teacher!) who had a love for and background in math. She was rewarding the tough reading work with doing some new things in math (at the time it was algebra, he was in grade two)once it was done, and it worked spectacularly. I'm always encountering this weird attitude that enjoying math is almost pathological (and now that we do have a diagnoses I realize this might confirm it for some people) and it drives me crazy! I was one of the few girls in the national math competitions in my school, loved calculus, my uncle is a physical engineer (thermal dynamics), we have lots of scientists in the family circle, etc, so I've seen how great loving math can be. I also know that even when not 2E, mathematical thinkers aren't the norm in our society, so finding a role model who thinks somewhat the way your son does could really help!