If one can square it roughly with the facts, it's always better to stick with 'superior for everything' as this keeps things simple. But as you say, sometimes it fits reality to make troops superior in only one particular way. As for British firing, it's superior for giving fire only (same as Austrian artillery).
We at the "Alvechurch Templars" Club in Worcestershire also count British line infantry as 'superior' for firing only. To be fair to other players, a premium of 10 points is paid - so a standard battalion of British line costs 110 points.