I have been reading on this subject for a while, and if I am not mistaken: Thunder Hammers are usually used as Two-handed weapons because of the shock force accompanied by the impact. That is why the power field within the weapon only activates moments before impact to somewhat avoid the problem; however, some chapters favored the approach to allow some of their Assault Squad members to wield the Thunder Hammer with a Storm Shield so that it could absorb some of the impact energy, allowing it to be wielded with one hand.
Conclusion: Allow the player to wield the Thunder Hammer as a One-handed weapon in combination with a Storm Shield only. If the person loses the Storm Shield or does not have the Requisition points to get both then the wielder must wield the Thunder Hammer as a Two-handed weapon to make up for the impact power. As for adding double the Strength Bonus, maybe it is only when you wield it as a Two-handed weapon, making it a reason for others to prefer using it as such.
But what do I know, I am a relatively new Warhammer fan, and I also never played the game though I wanted to do so.