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?
Exactly. And no, not even a stresstest could reveal the real implications of 300MB blocks. Stress tests typically last some hours or at most some days. But most people forgot that in Bitcoin-based currencies all full nodes must validate the entire blockchain, that means - in a 300-MB-scenario - weeks of downloading and validating blocks with high CPU/RAM load. It's not only done downloading and storing it, that would be the least concern.
The only coin that is somewhat "proving" that a higher blockchain capacity is possible is Ethereum because of their high transaction load in the past months, but the differences with Bitcoin are not very high.