Well, this may be a matter of how this gets worded.

There is no increase in gain with a MV.  None. Same preamp, same preamp gain structure. 

However, it would allow you to get that preamp running hot without have to crank the amp up to ear-splitting levels.  So, you can get the most out of the gain available from the preamp, but choke it off before the power tubes.  It will have more of that saturated, buzzy preamp OD and less of the traditional Plexi crunch.  This kinda gives the feel of having more gain on tap, but all it really does is allow you to turn up the preamp to a level you often wouldn't have the opportunity to crank a Plexi to. 

So, all told, you are probably looking at less than JCM800 gain.  At best, about the same as a JCM 800.