By TWoK they're able to power phasers with auxiliary power, so I'd imagine that direct fed phasers was a brief flirtation. I went with rechargeable power cells in each emplacement because I thought that might let them work with reserve power, but I'm not sure what I think.
The assumption that underlies all of the phaser explanation is that phaser power is actually somewhat special. It takes the special prefire chambers to convert whatever power source's energy into rapid nadions and then fire them. Hence having a big number of chambers is useful.
As for "megaphasers" I've always been a bit dubious on their existence. Reliant
's phasers didn't seem all that "mega" to me when firing on Enterprise
(in fact, Enterprise
's main phasers seemed more powerful) so I'm not really sure if they're as powerful as people suggest. One idea is that they might actually be fed directly from warp reactors or even the impulse engines, so have a different power structure than the primary hull phasers, making them more effective in that regard. Either way, I think it's safe to discard them as being substantially more powerful than a typical phaser emplacement.
Now, I toss that completely out the window by making the 24th century versions of Miranda
fit those emplacements with Defiant
style pulse phasers (which seems to me, to be a pretty reasonable assumption).
my assumption would have been that the phaser "arrays" underneath her saucer were surreptitiously upgraded from the standard Type VIIIs to either Type IX's or Type X prefire chambers, allowing them to have a higher overall output compared to a standard Excelsior