I wouldn't say that's true at all. In a car environment, oftentimes, the car's interior easily makes more of a difference than the quality of the components. It's not uncommon for people to ask why their stock speakers sound so much worse when they just spent $500 on a new hu. The answer being the built in processing in the hu, built for those incredibly cheap speakers within that car.
What a speaker is capable of is VERY install dependent. While many agree with this in the SPL arena (single small woofers hitting 160's), this is also true in SQ. A aftermarket speaker may have a smooth responce inherently, whereas a cheap woofer may not, but if the car gives you a nasty peak at 2k, well the cheap speaker with a DSP can EQ out not only it's own imperfections, but the interior issues too! You can't beat a good isntall, and a DSP is a very powerful install tool