The YJM is a quiet pickup, this is how Yngwie likes it, he allows the amp to overdrive the signal and prefers to have a very clean signal from the guitar itself. It's no surprise that in your RG there's a big difference in volume. Tech specs of the YJM on the Dimarzio site say this pickup has an output of 93. Look at a high
output pickup such as the Fast Track 2, for example, and you'll see it's 321. Output can vary greatly, and you should bear this in mind before replacing a pickup.
I would imagine the output of your RG's humbuckers to be somewhere around 275-300. I'm personally gonna fit a Chopper to my RG, I think. Output is 260.