Well, I don’t have Valeria for comparison, but as the forum’s resident Thorne defender, I’m gonna say this would be an upgrade. Even after his latest buff, Thorne’s special by itself doesn’t leave quite a big enough dent in most defenses, especially now that they are mostly heavily emblemed. However, when stacked with attack up and defense down, you really start to notice, and as you correctly point out, charging at average speed (i.e. at the same time as Kiril and Grimm) is a feature, not a bug, of using Thorne. Honestly, he’s part of my 3-2 blue stack more often than not these days, but my mono blue stack of C. Kiril, C. Sonya, Thorne, Magni, and Vela is pretty deadly. And Isarnia is on the way, so I can switch Kiril back out of costume.
If you are going to emblem Thorne, I recommend going down the attack path. He’s already very sturdy, and with the paladin talent and the bonuses you’ll just kind of pick up along the way, he’s a hoss. Make that attack hit even harder. That’s what he’s for. I have Thorne at +16, which means I’ve taken the last attack node available to me, and his attack is now at 732. At 480% to the center target, that’s nothing to sneeze at.
I agree that Thorne’s costume looks very usable, but I’m honestly hoping to pull it just for the stat bonus. With a 5% boost to attack, you’re up to 769, and with the mana boost, you could probably get him to charge in 9 tiles with a decent blue mana troop. That’d be pretty dang sweet.