Re primary a good one is an advantage but it is far easier to make up for a good primary at home than a good secondary and he only has two years left before he will moves to secondary anyway.
Tolstoy, I think I would look at the situation in a slightly different way. As you DS has dyslexia, would it be better to establish which schools are stronger at supporting such students and helping them to gain the results they are capable of? Perhaps all schools in Bucks are exceptional in that respect but I would be asking the question to be sure.
I note that you have mentioned that you DS needs to catch up with his English. If he gained a place at a Grammar school is the pressure of catching up and maintaining the same level as his peers likely to affect his self esteem?
I would also be inclined to look for a good primary for your other DC as Grammars do not offer sibling priority so they are going to have to gain a place off their own merits. Granted you can supplement their learning at home, however, if you are having to tutor them for both their SATs and the 11 plus, it will increase the workload and the pressure.