I have a 2010 2.5 on an engine stand in the basement, and my well used 2005 2.3 on a stand too. I'm not sure about block mods, I'm listening.
the 2.5 head has bigger intake ports and bolt pattern, so an adapter is needed to mate 2.3 intake to it, ask the Mazada guys about it.
that's why you would put the 2.3 head on. That and get rid of the cam phasers.
there are no Rwd 2.5 intake manifolds. unless you use the Brazilian intake and fabricate a throttle body/IAC mount.
the geared balancer unit is self contained, and oil fed. the issue is that it makes the oil pan deeper.
that would be a big problem on a 4x4, not so much on a 2wd.
the balancer unit can be removed, the oil feed needs plugged. then a 2.3 pan can be installed.