I shouldnt be posting this here, but it's official: Laevateinn is stronger than Lavis Cannon.

Here's the complete grind info from the JP Wiki:

魔剣レーヴァテイン (Demon Sword Laevateinn)
クバラ (Kubara)
PP:382
攻撃力:901 (ATP)
命中力:387 (ATA)

Grind---PP---PP Increment---ATP---ATP Increment
+1-----422--------40-----------991----------90
+2-----429--------7-----------1006----------15
+3-----435--------6-----------1016----------10
+4-----439--------4-----------1024----------8
+5-----442--------3-----------1031----------7
+6-----445--------3-----------1038----------7
+7-----449--------4-----------1046----------8
+8-----455--------6-----------1056----------10
+9-----462--------7-----------1071----------15
+10----502-------40----------1161----------90

As you can see, Laevateinn follows the same pattern as Elsydeon, where the first and last grinds have a high increase in both ATP and PP. No wonder japanese people are overpricing them right now.