Yes, you want the R2 volume mod. If you are currently needing to dial back for cleans, yeah, that probably won't change for you unless you are doing something odd right now like running Volume at 10 to make R2 louder.
I would say that if you are mostly a high-gain player you don't need and may not even want the reverb mod-- not because it's expensive, because it's not, it's one resistor, but because it can be a bad compromise.
The Mark III has no "off" setting for reverb; there's just more reverb and less reverb. (Apparently this is for noiseless switching reasons...?)
Anyway, without the mod you have a choice between "very little reverb" (off) and "some reverb" (on). After the mod you have a choice between "some reverb, not a whole ton but still enough to make high-gain playing kinda smeary" (off) and "enough reverb that anything except clean sounds kinda bad" (on). If you want "no reverb" you have to set the reverb control to zero, which means you have no reverb either way.
So, anyway, for most high-gain players where what you want is "reverb on my cleans and no reverb on my high-gain stuff" it doesn't actually work, and the mod can make it worse than the stock situation.