First thing, take everything I say with a big grain of salt since I know very little about audio and even less about car audio...
It seems to me that getting good bass from stock speakers or even upgraded speakers in a car is a complicated matter. My (limited) understanding is that (sub)woofers typically have two basic designs: acoustic suspension (e.g., Cambridge Soundworks and Henry Kloss) or bass reflex.
In the first instance, you have a woofer mounted in a sealed box; in the second the box is open with air exchange occurring through a tuned bass port. In both cases, the volume of the box and/or the size of the port play an important role in sound reproduction, and the speaker enclosure is generally made from some sort of tough, hopefully non-resonant material.
If you've ever disassembled a car door you know that it doesn't fit any of those design criteria. Lots of holes and lots of resonance, especially in the sorts of econo-cars I've driven all my life. Our 2014 Leaf is the best car I've ever owned; other have included a 2010 Honda Fit, 2007 Toyota Yaris, 1998 Hyundai Accent, 1990 Dodge Shadow, and multiple 1980s-90s Pontiac Grand Ams... I'm sensing a pattern of being poor and frugal here.
So, while I have no doubt that changing to better speakers can improve overall sound and bass reproduction to some extent, I think you have to have realistic expectations. If you want more bass, the easiest way is probably to add a dedicated subwoofer (which will fit the design criteria outlined above). I suspect that you will get more (bass) bang for your buck via a subwoofer than through more expensive speakers mounted in the factory locations.
I have read about cars with fantastic factory audio systems, but any one of those cars costs (way) more than all the cars I've owned in 25 years combined. When you buy a Nissan Leaf I think you're paying to be at the (relatively) leading edge of the transition to EVs. For convenience they include an audio system, but it's not the point or a selling feature of the car.