In other words, Dash is claiming to be able to safely scale to 300MB blocks but so far we have no prove of it. Is there a Dash testnet? Have there been any stresstests so far?
there is no incentive and nobody cares so no! no one has ever bothered to do a spam attack (or the pretty word: stress test) dash or any other altcoin. it is just not worth it.
the funny thing is some of them go down on their own under normal circumstances like what happend to ethereum. i can't even imagine what will happen when someone spams these coins.