The file sizes are the same between 750S in extreme mode to 900S in x265 due to the different compression types.
The main issue would be the heat. The 900S already has overheat problems - throw in an LTE module and you'll need a chimney installed in your roof lining.
BlackVue should consider their own LTE modem as an additional extra.
That way, it's not self contained and overheat, and can be bought as an extra, just like the various battery packs. Then it can be stored somewhere else in the car, and won't make the dashcam an oversized beast on the windshield.