At the moment, the Soul Gem System has three types of upgrades related to cultivation. The first one was obviously the [Energy Absorption Upgrade], which was already at level 5 and increased the rate of absorption by 25%.
The second one was related to the twins' Ying Yang Starlight Body Cultivation Technique, [Starlight Energy Absorption]. However, this one was still at level 1. The reason for that was that the twins never saw a need to upgrade this one. Their Starlight Cultivation was able to keep up with the Divine Energy cultivation. If anything, they usually reached the limit of the Starlight Body Cultivation before they could break through to the next level.