Good idea, but I see the permanant positive effects on behavior causing problems with long-term forts. Over the years, you accumulate so many good mood bonuses from 'artifact speeches' that it becomes downright impossible to lose a fort to tantrum spiral, even if you try.
The first logical conclusion that comes to mind is that if there are perma-buffs, there should also be perma-penalties (for example, your legendary fisher got dragged into the river by a now-named carp. Tragedy engulfs your denizens, and now they're all afraid to go near water!), or even some neutral effect that causes a behavior change that's neither inherently good or bad. It seems really interesting, but it throws consistency into question. Too many changes could present the player with a combination of bonuses/deficits that their dwarves become totally unpredictable, which gets in the way of things like megaprojects.
Of course, that's the worst-case scenario. If the likelihood of these changes is rare, and the changes aren't so drastic that it breaks the game over time, I think it could work... however, I'd want to explore other possibilities of game effects before settling on it. Maybe it could effect the number of migrants you get per year - I'd find it especially useful for offsetting the "Nobody even considered settling in such a death trap this year" message in a terrifying biome where I need as many dwarves as I can get!