I think
@BMW 1M is spot on: a DSP to get a good frequency response and decent stage and enough amplification to get satisfying output can make even inexpensive speakers sound pretty good. And
@ItsonlyaHONDA is right to point out that a measurement microphone will be the best way to get a good tune.
Even if you don't have "trained" ears, you will still be able to discriminate between a good sounding stereo and a crummy sounding one. It's immediately obvious. Maybe even more obvious in a car than in a home because most car stereos sound bad, but a good car stereo sounds fantastic.
As far as moving up the line, what you should be getting for more money is 1) smoother frequency response, 2) more power handling, 3) better breakup mode control, 4) lower distortion
All of those things make tuning easier and let you get higher output for more headroom (impact). You also often get better made or designed installation bits which minimize the amount of time you would have to spend faffing with fabricating stuff. Now whether the car specific features are worth as much of a premium that manufacturers levy versus drivers from Scanspeak, Seas, SB, etc is kind of an open debate where the fashion on this forum seems to change every couple of years.
Anecdotally, I just sold some Revel M16s to a local enthusiast. I plugged them in and he asked to listen to his reference Santana track (ugh). His impression was immediately positive "wow these sound great for their size" and he agreed to purchase them. I then plugged back in my M106s and he asked to listen to them. His face fell as he said "holy **** these are good sounding speakers." These two speakers have very similar spinoramas and predicted in room responses, but the M106s have better directivity matching between the woofer and tweeter, and have significantly lower distortion across the board as well as more power handling.
The differences are audible, but IMO not worth worrying about until the fundamentals (smooth direct and reflected frequency response, proper crossover summing, proper time alignment and level matching, sufficient sound treatment, reliable install) have been sorted, which is difficult in a car.