If you batch sparge - in a high gravity beer with a consequentially larger grain bill... you must get lower efficiency. It is the nature of the process.
You lose say 1.1L of wort to every kg of grain yes?? Well if you have twice as much grain and the wort is twice as strong... you lose twice as much liquid which is twice as strong.... so thats 4 times as much sugar you are throwing away with your spent grain. And every litre of deadspace in your system sucks out twice as much sugar. You can sparge more to compensate, but you will end up with a higher pre-boil volume that you have to boil away.
As for fly sparging - well with a given L:G ratio.. you use less sparge water to hit your pre-boil volume, so you are likely to get lower efficiency due to the reduced sparge, and there is every possibility that your system just physically doesn't work as well when it has more grain in it. The run-off dynamic is going to change with grain bed depth... and you double your grain bill, you double your grain bed depth - so its bound to change. And the change is usually for the worse.
Pretty much everybody gets worse efficiency going from low to high gravity beers. Its just something to tweak over time and to anticipate so you hit your targets.
TB
