In the last 5 years I have submitted about 3,000 cards to SGC, about 2,000 being crossovers from PSA. FWIW, my experience has been the following:
Crossovers from PSA holders, depending on the type of card, run about 60% crossing at the same grade, 10% half point bumps, 2% full point bump. Over a quarter of the cards did not crossover. Either those cards did not meet SGC standards, showed evidence of trimming, or SGC could not see the edges in the holder sufficiently to give a grade. The vast majority were not crossed for the first reason.
SGC cards with the old holders and labels (non guaranteed cards) crossed over at the same grade about 50% of the time, and I cannot recall ever getting a bump in grade. SGC cards with current holders and labels (guaranteed cards) received very few bumps. Note that these were SGC 60 and higher where SGC has already had grades reflecting a half point, SGC 70,80,86,92. I would expect submitting lower graded cards, SGC 50 and below, would result in more bumps because half grades were not always available until recently at these levels.
If your question is alluding to reviews and not crossovers, I would expect a very low rate of bumps. You are in fact asking SGC if they under graded the card. You would probably have more success getting cards reviewed in person where you could lobby for your position. Again, most of your successes will come with lower graded cards where the half point bump didnt exist before.
I am somewhat bothered by the fact that you mentioned you had a group of cards that you thought should be bumped. I am not aware that you have any special expertise in grading cards. I could be wrong.
I would also mention, that the times i have felt a card was under graded by SGC, I broke the cards out and resubmitted them at a later date. I was always amazed that SGC graded them right at the same grade as before. I know many will give exceptions, but that has been my experience.
Remember the axiom, that over time, every card will eventually reside in the highest grade holder possible. In other words, assume that any card you buy has already been reviewed and resubmitted by the previous owners who like you want higher grades.