That wouldn't increase the rear chamber volume that much. If you would include the space that the high driver is mounted in that would increase the average rear chamber volume three-fold. You would then have a rear chamber suited for the Beta 12.
However it would be a bit like putting big spikey tires on a city-moped, it's now better prepared for off-road but isn't quite a dirt bike yet. By that analogue you would need a better motor as well, which is what the Beta will still lack, after increasing the rear chamber (also the Xmax isn't that happy to support the increase in chamber size).
Personally I would advise you to look for better drivers but it would hurt a little less, if you'd still would go for the Beta's now