None of the smartphones today have dedicated GPS chips in them, they only have a GPS capability that stems from byproduct features of the modem chips.
That's not true. GPS functionality in phones is done either with a dedicated chip, or with a dedicated GPS receiver within a System-On-Chip (which is effectively the same as having a dedicated chip).
Because of that, the GPS capability is very limited and poor
Huh, that would be news to me. In fact, I find GPS performance of phones astonishing, especially considering the fact that you can't just reserve the same size of antenna as in e.g. nautic navigation systems.
you only receive one basic L1 signal, whereas if there was a GPS chip — it would also receive L2E as differential signal, or SBAS - a correction signal, thus drastically improving accuracy.
As far as I can see, embedded GPS chips basically support all features of civilian GPS. Your claim has no backing.
My phone's GPS functionality has excellent differential GPS capabilities, for example.
Question is would they ever do it, or would it be too big of a hit on the battery, some of the chips require pretty high voltage for some reason like 3V.
Since they do, the answer is clear: Using the GPS circuitry / antenna does cost some battery, but every phone has GPS and thus it does work.
3V isn't per se a high voltage, even inside a smart phone – it's significantly below battery voltage, so easy and low-loss generatable from the battery.