Have you tried taking a GMATPrep yet?
After taking my first GMATPrep, the majority of the problems I encountered were def. easier than MGMAT and anything on the OG Diagnostics.
There were a few problems that could be considered MGMAT level, but nothing too extreme. My last Q was a 48 so I think I saw many of the tough problems.
However, many of the OG probs are not representative of at least the GMATPreps. Make sure you do the higher number problems. Most of the time, these problems are of the more difficult problems in the OG. Around 600-700. Very last problems can be considered in the 700-800 level.
If you shooting for a really high Quant score, then your going to have to use something outside the OG such as MGMAT or GMATClub Challenges