We might argue that the phasers on the roll bar were more powerful ("larger caliber") than the ones on the hull. However, their effects on Kirk's ship certainly don't show this. And Khan didn't wish for maximum effect anyway, he merely wanted to wound Kirk's ship.
I think it was even stated that under normal conditions the enterprise would outgun the reliant; it was the sneak attack that did them in.
Quite possible, even though this isn't actually ever stated on screen. However, the Reliant
model as completed has at least as many weapons emplacements as the Enterprise
model, and twice the number of those mean-looking torpedo tubes, very prominently positioned. It would seem that aggressive looks were a very important aspect of Reliant
design - even if we argue ex post facto that Starfleet designed the ship to be convertible
for extra aggression via the roll bar and wasn't quite that militant in the basic configuration.
Certainly the idea that the Reliant
would have been a dedicated research vessel was dropped from the final movie, and we instead get the impression that Starfleet decided to do the Genesis testing with the help of a potent warship. Perhaps not Calypso
style at all, but more like how the first atomic bomb was carried to the front lines aboard the top-notch heavy cruiser Indianapolis
in full combat readiness.