Not too sure. Leviathan has a lot of gas:
- Baam thought it would be a sin to the world to unleash him from the Suspendium.
- Leviathan boasted about having possessed strength that towered above everyone, including the Ancients and even the Irregulars (though he got punished and sealed by Traum at the end).
- He one-shot the Odd-Eyed Cobra (BH Tier) while still being sealed away and as collateral damage.
- He gave Baam enough power to make Dumas (Regent Tier) go all-out, and even give him a bit of damage through that form. This is while still being technically sealed away. Ergo, a small fraction of his power (or should I say Traumerei's?).
- Traumerei gave up on trying to control it, while he could control Valhalla.
I can see a fully unsealed Leviathan ending up as stronger. Quite frankly, I don't think Valhalla will end up that close to FH level anyway. Traumerei should only be FH level when you take into consideration all of his abilities. Basically: Traumerei + all his countless Shinheuh (including Valhalla) ~ FH level. Among the countless Shinheuh, you need to take into account that they have infinite lives as long as Traum is alive + can be fused to further enhance their power. Valhalla on his own would only be a fraction of all that (a meaningful fraction most likely, but still a fraction).