Youre effectively increasing the slots of battleships like the tempest, raven, rokh. This isn’t a trivial thing. It means balancing them from scratch. Enter the buffer fit tempest with tackle to boot 
And your current stats for an XL shield extender can fit quite happily on some BC’s like the drake, effectively removing the need to fit two large extenders and letting them put on an XL extender and a web. Or (god forbid) a hurricane that can fit one of these with an invuln.
Where’s the trade off?
This has been proposed a thousand times, but its not going to happen because it basically re-writes buffer fits for BC’s and above. Instead they added capital sized buffer mods.