First of all, thanks for the support and I'm glad you enjoy the videos. I can say I understand his low grades from a performance perspective when it comes to BR/PFF/etc. He's an average starter with upside in his ability to help with playcalls and adjustments on the field. He's (likely) never going to be the top LB on any team and there were certainly plenty of plays that showed his deficiencies. How low he ranks is really a matter of opinion. BR's NFL1000 listed him as the 31st ILB even though he played SLB on base and ILB on nickel (though Allen rotated most of his LBs everywhere). PFF had him as #80 which is lower than I would have him. I will say his coverage ability left a lot to be desired (I had higher expectations for this studying him the offseason before).
The key for me is that he was an upgrade over what we had. Even if he is "below average", New Orleans had worse than that at LB for a couple of years. Either way it was an improvement. I feel the same with Craig Robertson. I don't think Robertson is good enough to be a full time starter as a LB. I love him for what he is. A Leader (ST Captain) and quality depth piece for when someone goes down. In the end I only care about if the team has improved and I think the LB group has.
As Ellias said though, if you're confident in T'eo or Klein as full time starters in the middle, you don't go after Davis. As for making a film study on him, I might have to do that!