I recently had efficiency issues with my Baltic Porter that was supposed to have a 1.089 OG and had 1.065. I boiled up 2 lbs DME in just under a half gallon of water and added about 24 hours after pitching the yeast. My gravity sample yesterday tasted fine, but I still have some time to go till it's finished. If you are way off of what you were shooting for, it can be done with some risk. If you are relatively close, a summer session brew wouldn't be bad either.