There is a lot of math going on in 4E, people are saying even small changes break things. A DM is not encouraged by 4E to do arbitrary changes.
Listen, we're talking about a level 15 solo vs a level 3 party. The level 3 party is going to die. You're telling me we shouldn't change a stat for fear of, what? Unbalancing things? The level 15 solo vs level 3 party fight might be unbalanced???
Unwinnable combat situations are often appropriate. If the party decides that the dragon who wants to send them on a quest should instead be fought they should get the dragon that they, insanely, want to fight.
I do not think it is appropriate for the players to fight the dragon they want to. I think any and all NPCs including monsters are the DM's abode. If a player says "this monster should deal X damage, not Y", I think he is overstepping his boundaries.
Note: the DM is pitting a level 15 solo against a level 3 party, and I'm saying the players are overstepping their bounderies, hehe
8 levels later when you WANT them to finally fight the dragon they say "no fricken way, it's breath does XXX, we'll lose!" and you, as a DM, are sitting there going... "damn."
Consistency is a bitch.
As noted in an earlier post, my belief is that 4E is a relative system. A level 15 solo is that strong for a level 15 party, but much stronger for a level 3 party.
This being said, I encourage players to avoid meta-gaming by saying "no way, its breath does XX damage, ..." Though I get your meaning. If he were to say so, I'd reply (after the please avoid metagaming thing): "you know, when your PC was inexperienced, he had not learned how to avoid the brunt of a powerful adult dragon's attack. Now that he has raised high above most people in power, it is likely that this attack would not be as devastating to him". I.e. a level 15 solo might kill a level 3 with a single breath weapon, but to a level 15 he'll do what the book says.
All this being said, as mentioned in earlier posts, I'm not supporting killing PCs with a single breath. I'm supporting DM free will. It's a game about imagination. If you can't think out of the box, then stay in; however by staying within the confines of the books (or box), you can't pit a level 15 solo vs a level 3 party because that's what the DMG says. This is the inconsistency I was trying to highlight. Either you accept to deviate outside of the rules, or you don't. If you do, then at least do so in a way that suits your game. In the case of the OP, I understand that to mean that he should have the dragon deal more damage on a breath weapon.
Sky