I never noticed much difference with elderseal, I would say to consider it like a bonus and not something to shoot for. It's a hard thing to determine how much effect it has. I have the Nerg great sword which has high elderseal, but I can't really say how much it helps when compared to the Wyvern Ignition, because the Wyvern Ignition outclasses it in damage. If you haven't beaten the elder dragons and are struggling with them, it might help you survive a bit more, but my gut feeling is that it's probably not that noticeable.