From reading this board, GMATprep appear to be the best indicator of actual test performance out there. That said, my score jumped 50 points on the actual test (710 on both practice tests). However, some have had the experience that scores drop on the actual test. I would take your scores with a grain of salt. I wouldn't predict your score on the GMAT to drop 100 points, I think you could be confident of getting 700+, probably higher. If all you have is the OG11 and you're doing rather poorly on SC questions I would look into getting a prep book that goes over grammar and common errors found on the GMAT. People on hear talk highly of manhattan prep. I think they have a book specifically on SC. As for practicing, I would definitely focus on OG11 questions and figuring out a system to solving the SC questions in this book. Make sure you're studying the reasoning for the answers as this is the only way to learn to solve any SC question. The OG11 are very much like actual GMAT questions and there should be plenty in there to keep you busy.

Good luck!