It is also somewhat questionable to ask someone to prove a negative.

This isn't true at all. The negative is also often seen in the form of a null hypothesis. "There's someone in the fridge". "No, there isn't". Here is a case where we can prove a negative. Some negatives by their nature can't be proven, but as a blanket statement it's false.

You yourself mentioned that benchmarks were worse - this is proof of the negative, and you could have cited those benchmarks in your argument.