I recently brewed a major Imperial IPA and also noticed some loss of efficiency. Luckily I was warned about it in advance, so I lowered by estimated efficiency from 80% to 70% -- and I hit my target within 0.001 points! Some things that also help, lautering very slowly (I think mine took 2 hours), starting out with more wort and boiling longer (I did a 2-hour boil starting with 8.5 gallons instead of an 80-minute boil starting with 7.5 gallons), and occasionally raking the grain bed about halfway down. It's a lot of extra work, but it should be worth it.