Short answer is yes, there's a quality difference. In specific as you raise the volume an amp will actually not have to work as hard to produce the power/volume ultimately that the HU will and will not go into whats known as clipping. Clipping is what happens when the amp (on board the HU, or external) is no longer able to reproduce signal at the requested level perfectly, and so the signal wave is truncated or clipped. there are very good articles on this that go very deep if you want to dive into electrical engineering world.
anyways, basically an amp can produce signal and volume more easily than the HU. Mind you just about any after market head unit will produce more than a stock.. although the FRSs stock is pretty darn good, so in the FRSs case I'm not certain that's true.
so compare apples to apples most after market HUs produce about 15-22w RMS (the RMS is a better measure than peak power output, for reasons Ill let you lookup if you care to) where as the 15A alpine and pioneers produce 90w RMS to 2 ch, or 45w RMS to 4ch. and what @
B-R-Z is running produces 75w RMS to 4ch, meaning he's capable of pushing an average of 3.5x the power that the HU alone could.
edit: as an addon to this story I have had 2 different friends tell me they thought the speakers in their cars were blown out, when for one he needed an aftermarket HU, he was running a stock jeep HU, and sony speakers which clipped pretty much constantly and for the other he would hear clipping in his tweeters at high volume and thought they were broken until I recommended he try out the alpine ktp-445u, and now he's happy as a clam.