Roxicodone and Percocet have the same narcotic -
oxycodone. The difference between the Roxi and Perc is the Tylenol, which is found in the Percocet.
Will the Perc work better? That depends on the person, honestly. They say coupling Tylenol with Oxy (or Hydrocodone) increases the effectiveness of the narcotic. Pharmacologically, I'm sure there is a valid reason to couple the two, but the reality is that i probably depends on the person using them.
So your Perc 7.5/325 is 7.