It has been tested that rank (not sure about class though) does not affect the amount of energy, from the ENG capacitor, required to boost. If you feel a difference, it's either placebo or you may have upgraded your power distributor with now a recharge rate faster than before.
Short story on my thought about rank A thrusters:
if you need highest boost speed, go for it; otherwise, rank D is fine.
Long story:
It was also tested (on a Vulture) that, in the middle of the blue zone, a 5A vs 5D gives only 0.5s improvement for a 360 turn. While it allows to fly faster within the blue zone, you actually end up with less sharp turn since the blue zone is now higher...
I personally would not invest credits and energy use to increase maneuverability by such a small amount.
However, boost speed is another story and, if you plan on escaping or outrunning your opponents, then a gain of 50m/s is worth every pennies.
I've done similar tests with a vulture doing 3 complete loops (pitch up + thrusters up) with engine at 50% with 2 pips to engine then I repeated with 4 pips to engine. I've tested 5A thrusters VS 5D thrusters.
There was a gap of 3 seconds (1 second / loop) between the thrusters. Plus with 5D thrusters you can save energy and I am able to mount another shield booster.
Later I will post the results of my test if interested.