Hi,
We do gf/cf for ds2, he's 8 and autistic. We didn't do sunderland test because there was a long waiting list and we wanted to get on with it. Ds2 at that time (age 4) was self-limiting his food to pretty much only gluten/dairy foods, which is apparently often an indicator.
We did casein first, and saw an immediate (within 3 days) improvement in listening and verbal skills.
After 6weeks? I think, something like that, we took out gluten. Took longer to see an effect, a few weeks, and a less dramatic effect, but I think more profound, he felt more 'present'. Obv, it's hard to say, because by then he wasn't having either, but we were advised to take out both, as you say. AiA also advised removing sweeteners, esp. aspartame, and monosodium glutamate, which we did. They also suggested a lot of things that we haven't done.
When we startd we went gf/cf as a family, but after a year the rest of us started having some gluten and dairy again, but we have to be quite careful with different areas of the kitchen, not putting toasty knives into the honey etc, as tiny bits seem to have an effect on him.
I'm glad we started pretty strictly then relaxed, as I do't think we'd have seen the effects as clearly, and we now know what we can get away with. Eg., he can eat chips when we're out, although most places have the kind that have a tiny coating of some kind of flour to stop them sticking, without too much effect. But he can't have most ketchups, either because of additives or because most of them sweeten with wheat syrup rather than sugar, without going loopy, so we have a bottle of heinz wherever we go.
The only supplements we can get into him; he's very opposed to them! are cod liver oil on home-made fishfingers (which I wouldn't advise you start making, they're a pain, and these days you can buy them, he just won't eat them), and biona do a powdered multi vit and mineral which we mix into ketchup. There's nothing else we can get him to take.
It does seem daunting; I put off trying it for months, but it's really not that bad, esp if your dc eats a wide diet (mine doesn't!).
And absolutely worth it in our case.
Good luck!