basically, the fusions are pretty good bang for your buck.... they are not cheap and nasty and perform ok for the cost. Of course expensive high quality speakers are better but I personally don't want to spend a fortune because a car is a "vulnerable item" .... by that I mean all it takes is a car thief or another numbnut driver crashing into you and your car is gone/fubar and you have to start again.
In regards to bass output you will never get amazing bass using just the door/headrest speakers but a lot of it comes down to what you are listening to. For example, listening to the cult or supertramp sounds fine without mega bass but dubstep and drum n bass sound stupid and pointless without it.
If you don't want to install an active sub then I would agree with your choice of the fusions in the headrest, but I would also consider looking into something like dynamat or a similar product to dampen outside noise .
http://www.dynamat.co.nz/
I hope this helps a tad
-=edit=- by the way, the only reason I went JVC was simply due to the cost at the time of purchase, they were on special and were within my desired output range. I think I got all 6 speakers for less than $100.
oh and one last thing, pop the door speaker covers out and behind the metal grille is a fine black woven material. rip that material out and pop your covers back on (I used a craft knife to do a tidy job) that mesh stops a lot of high frequency sound and will help your door speakers sound less muffled/muddy.
Due to the mx5's cabin design you will never get as good quality audio as a 2013 sedan but you can get pretty damn good if you are willing to put the effort in.
