Definitely something I was worried about too. I did some A/B testing with an actual espresso machine and didn't notice a difference. I also looked up some paper

Definitely something I was worried about too. I did some A/B testing with an actual espresso machine and didn't notice a difference. I also looked up some papers that had CO2 solubility in water as a function of temp and pressure and at the brew times/pressure/temp we're at, there's very little being absorbed into the water. Certainly not enough to affect taste. I use an air compressor to drive it now and haven't noticed a decrease in brightness either.