PEACE it may also depend on whether or not you ultimately end up getting put on the ER morphine, but like seiko said the APAP is very likely doing something. However, I'm not as certain that you're better off staying on the Percocet. You can easily add OTC acetaminophen to an IR oxycodone regimen. Right now you're somewhat forced to take the same amount every day because it's inextricable from your pills (save for a cold water extraction, which is entirely unnecessary in this situation). Right around 2000mg of APAP per day isn't terrible or anything but if you don't actually need it with every dose then there's not a lot of sense in taking it with every dose. That's all.
Anyway, I totally agree that APAP can be really effective for back pain and there's nothing wrong with a reasonable APAP regimen under a doctor's care. I just wonder if it's necessary to be taking it every four hours merely because you're taking the oxy but of course I can't answer that question for you. I just think if your doctor is willing to switch you to plain IR oxycodone then you could have more control over the amount of APAP you are taking by adding it via OTC Tylenol or generic acetaminophen. You might even be able to get away with taking less oxycodone overall if you were taking a similar amount of APAP in between some of your breakthrough opioid doses instead of concurrently. To be clear, I'm not saying you should take more acetaminophen... just that it may benefit you to take it on a different schedule from the IR oxycodone.
All of the above is under the assumption that you're comparing the same daily dose of oxycodone, or close to it. It's also no good if you don't actually add the APAP in, so it certainly depends on what you're willing to do and how well you are about keeping track of multiple medications (again especially if the morphine is added, too).
But... as long as you're not exceeding *4000mg a day, which you're not, it's pretty much six in one and half-dozen the other. It might be worth getting your liver numbers tested every so often if you end up staying on the Percocet on the off-chance your liver is overly sensitive.
I noticed that you've started a number of similar threads lately about your situation and the switch you may be making. I don't believe you've made any strict duplicates or anything but bear in mind that each topic you post about only needs one thread. If you have questions that are along the same lines of those you've already been asking or otherwise closely related to the same situations, you may be better off posting in one of your extant threads. I'm sure almost anyone who has so far responded to your thread(s) about your current pain control situation would have been able to help with this question and that puts it all in one neat place for you.
*it drives me crazy when people willingly take too much APAP. Taking 4000mg every day for any real length of time is still a large amount, and probably not the best idea unless explicitly prescribed by a competent medical professional.