In spending only half a billion in upgrades, I am nowhere near an expert in this; however, I have found that A rated is not always "best". I think A rated tries to be best bang for the mass, but often the power requirements make it a trade off to using a lesser rated component.
I think of it like this:
E rated - demo version, introductory offer but the least efficient.
D rated - mass management golden child, but only slightly more effective than E
C rated - middle of the road, lighter mass, moderate power needs, more efficient than D but not as good as...
B rated - bloated mass, more power, more bang for just under the top end component
A rated - same mass as C, power hungry, but the most oomf of the components.
I always upgrade life support to D and let it sit there to stretch FSD as far as it can go and still save on power. I work to get top end FSD for the range. For PP it depends, I find sometimes the A rated PP, a class under, gives a better output than the D rated component above it, saves a little more mass, and still provides better heat management. I think it's all about personal play style and the trade-off when outfitting. I don't generally find in my play style that solid A rated components make the better ship, but it's up to the CMDR to make the best of what they can.