Cecily, lots of helpful thoughts there. I don't really know the local primaries other than ours, which isn't that great IMO (they've only just really admitted that dd's friend - who effectively can't read at all age 10 - has any kind of problem).
I don't have any evidence to back this up, but I do suspect that working in two languages (they alternate 'english week'/'welsh week' from yr 3 onwards) can't help those who have problems, and certainly dd who was an early reader never actually got any English phonics instruction at all at school. I don't know though how things work in other primaries.
Definitely the Welsh medium secondary creams off the more academic and/or middle class kids - very much the same pattern that you'd expect to see in an English town where there's a C of E secondary & a community school.
"Will being in a school with so many poor readers really effect your child? Children with SpLDs can still be intelligent and contribute well in class. " I guess that's the no. 1 key question!
Did I have any previous doubts about the school . . . yes, it has undoubtedly been through a very rough patch (very unpopular HT, lots of teachers left, HT then left, acting HT for some time) though I'm moderately comfortable that it's on the up and the new HT due to start in September comes with a very good reputation. They recently did a grading of Welsh schools 1 to 5 (crudely, good to appalling) - this school was graded 4, the other 1 . . .
Basically, its just very hard to have the confidence to say that this school looks bad on externals (basically exam results), but actually it probably is the best school for my child.