Only problem I have is not being able to boil as vigorously (for fear of boil over) so my evaporation rate is lower so I can't use as much sparge water and hence only get 65% efficiency.
You can easily compensate for this a few ways:
1) Take a day to boil a full volume of water for an hour to measure the boiloff rate for your pot, you'll no longer add extra water and your efficiency will go up.
2) If you boil with the lid on, take the lid off and that'll increase your efficiency a few points as well.
3) Just add an extra 30 minutes to your boil. More water boiloff, higher efficiency.