Regarding Natsu vs the Sabertooth dragonslayers... they might be considered that powerful but please remember exactly how well would those two have fared against the kind of opposition Natsu picks fights with and I don't mean just the ones he loses outright but those that by all rights he should have lost but won.

Would either of those two have survived taking on Jellal for example?
How about Hades or Purehito?
I don't think so, yes they had that seven year advance but even then it wouldn't have made them that much more powerful since for those seven years Fairy Tail didn't have any dragonslayers to match against them even with the other guilds to really be able to judge just how strong they're supposed to be.
Would they have stood a chance against Laxus for example?

As for Gray vs Rufus well Rufus did give away that bit about memorizing three magicks and I figure Gray assumed to up the ante as he pulled out his usual tricks and do his imitate Natsu's talent for destroying the area around them!

Now I really am looking forard to them animating this!