1. There is no peer-reviewed data to support useful transfer and generalization from brain training for processing speed. I don't think it will hurt anyone, if your family can afford the time and money, but I don't have high expectations for benefit, either.
2. Other than WMI and PSI, his scores are quite similar to past testing. And PSI isn't a huge drop, to me. I do see that on his WISC-IV, his processing speed increased as the task became less fine-motor-heavy, and more concrete-familiar (images of realistic objects, rather than letter-like symbols). This may explain why his tests are completed on time, but his classroom activities are not. Classroom activities tend to me more novel, in the sense that that is when new material is introduced. By the time he reaches the test, it should have become more familiar. I would expect him to do well in social studies and science, as far as speed goes, because all of that is highly contextualized, which plays to his strengths.
3. WMI is harder to compare, as the WISC-IV consisted of two listening memory tasks, while the WISC-V has one listening and one visual memory subtest. We already know he is much stronger in verbal comp than in visual spatial, so it wouldn't be odd if he were generally better with listening than looking. That would easily explain the drop in WMI. (And btw, did the examiner substitute Arithmetic into the WMI, on the WISC-IV? Because that would also have given him a bump, since he's above grade level in math achievement.)
4. The WISC-IV PRI was split out into VSI and FRI on the WISC-V. I suspect the nearly 2 SD difference between the two was kind of cancelling out on the WISC-IV, making it appear to be only high average, when it was actually a Superior strength in fluid reasoning, and an Average mild weakness in visual spatial. Fluid reasoning refers to adaptive problem solving and integrative thinking skills. I always say, if you have to pick a strength, this should be it! Makes perfect sense that he's strong in mathematics, especially now that he's passed the math fact phase, and math instruction is increasingly about reasoning.
5. Has he been looked at by an occupational therapist, or for anything in the dysgraphic category of disabilities? His processing speed, especially with fine motor, and struggles with writing--especially where you note mechanics, sequencing, and editing--both would make me wonder about that. He has strong language skills, so pervasive struggles with written expression are even more striking.
6. If you actually think his reading is starting to stall out, you might want to investigate dyslexia, as well, as that would fit with the low processing speed, better performance on images than symbols, and distraction with reading and word problems. It could be other things too, but this would be one of my question marks.
