OK... So if we assuming for example a new 5A SCB module = two old 5As in all matters, except the limit of three uses per X minutes?
Well, maybe a 6A is akin to something like having three 6As SCBs. ie: Over 600Mj of recharge, and the same energy requirement too (as having three current 5As or 6As). That's enough to fully recharge the most tanked up Anaconda's shields from empty to full. And it's basically not changing any behaviour other than using less cargo space/mass and limiting the player to three uses in X mins.
Indeed, if you want, offer a class 7E --> 7A version too with continues up the scale offering basically yet more energy usage and shield recharging combinations of current SCBs, and above it too! Or simply give the 7 version about the same performance as class 6, for with more energy usage, and with four cells (instead of three). ie: Four uses per X mins instead of three!
I see what you mean. Because of the limited amount of charges per minute it is not so important to have hundreds of SCBs (you can't use them so often, anyway), It's only important that they are big enough to fill your shields. And with higher classes you can fill them more often than with lower ones. I like Rgconners suggestion a bit more. He does not solve the "stacking" problem (problem for traders to be competitive without losing too much cargo space), but spamming becomes more difficult and the ships can keep their unique strength and weaknesses.