It's part of the equation: high gravity beers have less sparge water and a smaller ratio of water is boiled off. High gravity beers suffer in efficiency, that's part of the game. How thick do you mash in? I like 1.1 quarts per pound to free up as much sparge water as I can. Are you fly sparging or batch sparging? Do you mill your own grain? I get 89% efficiency on beers up to around 1.075, closer to 75% efficiency on my big ales.