We have a 30- to 40-point spread between DS7's PRI (which is his DYS-level score) and his PSI and WMI, and I see the bottlenecks quite plainly.
After a dismal start to 1st grade, we pulled him out for homeschooling. We started the year last year doing a lot of mental math, and he was miserable! It took him so long to think through the problem (because he's not any faster than your average kid his age) that he would forget what he was supposed to be doing (because he's got a memory not much better than your average kid his age), and so he'd mess up the problem. I thought he was dawdling and got angry. It was annoying to me and frustrating to him. Lots of tears...
Even before that, he was always extremely slow on the "easy" work he had been getting at school. It made it much harder to work with the (already GT-resistant) 1st grade teacher on differentiation. She just didn't see why he needed it, since he couldn't finish the work he was given...*sigh*
After the WISC results came back, we did much less mental math, choosing instead to write down problems and work so he could see it. We still do some mental math to strengthen his relative weakness in those areas, but never enough to get really frustrating to either of us. Math time is a lot more fun now!
I really do feel like the bottleneck acts with him as a sort of learning disability. Not literally--he's not IEP material or anything--but it definitely affects the way he learns and the way material must be presented to him. I always say that he's deep but not fast.

Here's a practical example of his bottleneck: though he understood the concept of multiplication and could multiply long numbers last year if you gave him enough time (LOTS of time!!!), he's no better at memorization than an average 7yo, so I'm not pushing hard on the times tables. In terms of conceptual understanding, he's like a much older child. So we're doing a lot of visual math that doesn't require much multiplication--geometry has been the biggest hit!--and we do just a sprinkling of multiplication work every day to teach it gradually, over time. Dice games have been good for that. Also, we worked on perimeter with pattern blocks, and he figured out that he could count the units used on one side and multiply by 6 to find the perimeter of a figure he built. He kept adding to it and refiguring the perimeter, so it was a pretty good day's work on the 6s!

So this is a long tale told in the hopes that it might register--or not--with what you're seeing with your child, Tizz. Maybe seeing someone else's experience with a bottleneck can tell you if this is what's happening with your child or not?
I hope so!
