The big lottery at IAH is the arrival of the other European aircrafts, and who ever gets in first means their passengers get a good run. Actually run is a good word, it can be quite a distance from the BA gate to CBP, along some long corridors, so you can perhaps gain some ground there if you are fit enough and not too weighed down by bags. On the other hand there is a hand baggage only "One Stop" channel at IAH . This is a special channels that avoid customs, but not if you have checked baggage too. It can be used by transit passengers too, even if they do have checked bags.

IAH isn't too bad - not the best, not the worst, 30 minutes would be a long wait there. And since the big issue - which bird is first - is outside your control then there's not much you can do other than go HBO. Your reference to T5 is interesting, as you will know, experiences here vary from "good" to "terrible", so I'm not sure what that proves!