Hi PTD1995,
A Q51 on a ClubTest is an outstanding performance, so assuming that these were all 'new' questions that you had not seen before, then this result implies that you have the capacity/ability to score at a high level in the Quant section on the Official GMAT. However, this result does not 'guarantee' you anything (in the same way that a professional basketball player who makes 20 straight free-throws during practice is not guaranteed to perform at that same level during an NBA game). The GMAT is remarkably good at giving each Test-taker the Score that he/she EARNS. On this practice section, you kept yourself focused on the tasks at hand and performed really well - so if you're looking to earn a Q51 on the Official GMAT, then you have to bring that same focus and willingness to work to the actual Exam on Test Day.
Since your Official GMAT is on Monday, during these last few days, you should limit your studies to general practice and light review (NO CATs and NO 'cramming') - you would be better served by getting some extra rest so that you can go into Test Day calm, clear-headed and ready to work.
GMAT assassins aren't born, they're made,
Rich