I definitely like the extra Stegadon with Prime Warbeast for sure. I'm just not sure that the once per battle special beam cannon ability is worth it for the cost of a battalion. *If* it's d6 mortal wounds then sure. A chance to truly nuke something. But if you're rolling a maximum of 3 dice, each of which could roll a 1 and get nothing, for a maximum of 9 MW once per game, I'm thinking it would almost always be better to just double-shoot the Bastiladon and hope for some mortals from your Engine roll.
If it's d6, then you could still get nothing, but now your maximum potential goes from 0-9 to 0-18, that's quite substantial. It's also not out of the realm of what other units can do in the game already. The Skaven Warp Lightning Cannon, for example, can overcharge whenever it wants and have a chance to do 12 MW automatically just based off a single die roll.