[aw1621107]

> I've always been curious as to how effective a hydraulic lift could be at reducing necessary launch weight.

I suppose you'd be able to approximate the effect by comparing the delta v needed to launch from a sea-level site (Cape Canaveral, Kourou, etc.) with that needed to launch from one of China's inland sites (e.g., Taiyuan, which sits at 1500m). I have no idea whether this data is publically available, though. I'd guess the bulk of your performance improvements would come from increased engine performance due to the lower ambient pressure (~0.83 atm according to Wolfram Alpha) rather than the increased altitude, since most of the energy is needed for horizontal acceleration [0]. The increased thrust would mean lower gravity losses, but I wouldn't be able to say how much.

[0]: https://what-if.xkcd.com/58/