Using 10 1/4 inch fiberglass insulation in a 6 inch space?
Yes, and no. This table shows you some idea, although it doesn't appear to show an actual number for 10 1/4 compressed to 5 1/2. Extrapolating, I'd guess you'll end up somewhere around R-22, which is still higher than R-21 for typical 5 1/2" batts.