The FDL is still the best combat ship in the game, the fact that the hardpoints have to be fixed is just one of the many sacrifices you have to make in order to get the best balance of physical size, maneuverability, vectoring thruster strength, hardpoint variety and distribution, and shield strength. I assume you're familiar with the basic pvp FDL builds? There are only three viable weapon loadouts, 4 short range rails and an efficient corrosive MC, 3 PAs and 2 rails (or MGPCs), and 5 PA. None of these weapons cares in the slightest about hardpoint size penalty.
When it comes down to it, these builds flown correctly, combined with the FDL's maneuvering and defense, are still unstoppable in pvp. There is no questoin that in the right hands the FDL is still the king, but you have to be able to really fly and shoot. This is not the case for the FAS. No matter how good you are, it can never reach the same capability of the FDL, it's mathematical, it doesn't have the thrust vectoring, the shield, or any other thing that compensates for its lack of those.
For fixed weapons, the FDL hardpoints are perfect, use any 2 mediums and the huge for your main damage dealer (use PAs), and you have perfect convergence, put the 2 rails on the right, again the reticles are basically on top of each other. This, plus the vectoring thruster strength under boost, are the main reason why it's still the best combat ship for pvp against other medium combat ships, all else being equal.
The Mamba is a great ship, I use it A LOT. I could even say I mained it for a while after it came out. Its biggest advantage when it comes to hardpoints isn't the size, but hte fact that they are all on top of the fuselage, making it a perfect gimbal platform that can hit anything it can see in front of it at any time. It has a notably weaker shield multiplier than the FDL, while being faster and having slightly weaker vectoring thrusters. It is not a match to the FDL assuming pilots of equal skill, but it's probably the third or fourth best combat ship overall.
I agree the Corsair's hardpoints are bad. Fixed only, except the two top mediums and the top large, but hey, it has insane dps output, great boost, great speed.
Yes, all ships have their soft concerns but the CD ships don't come out top of the pile at anything, or even close.