You can read the original XPS guide
at The Internet Archive.
I removed the modification because it does not work very well, as Andrew suggested. A major issue I have is that this modification reduces water volume in your pressure chamber. The other issues with precharging and the balloons breaking also are worth noting. I do recall saying that you could get CPS-like performance from this modification back then, but doing that well is really only possible by replacing the entire PC, not XPS.
In terms of evidence of performance increase, it's likely that the rubber increases the pressure, and this would increase the performance. Personally, I wouldn't bother, as XPs are not meant to be high performance water guns. As I've said before, you can't upgrade a tricycle to work like a monster truck without basically replacing the tricycle entirely. The same applies for water guns.