Charging chip does charging cap itself, based on voltage and what battery accepts. So, it's exactly as optimized for charging 300 mA battery, as 3000 or 10000 mAh.It just works as it should, out of the box, without any drawbacks.

As for charging from shutdown to full, for my ~3400 mAh battery, it's about 4 hours total, and 3 hours fast-charging*. Just keep in mind, that I use 1250 mA max charging current, as opposed to vanilla 950 mA. To do this, you need Pali's bme replacement beta, or properly modified charge.sh script.

As for balancing, dr_frost_dk is right, it doesn't affect parallel, only serial. in theory, we should use identical batteries of comparable age (thus comparable capacity and physical parameters), but parallel is so "forgiving", that people even did hybrids like 1400 mA battery + 1000 mA one, without drawbacks (still, getting about 2400 mA solid capacity and no drawbacks). Such things would be killer for serial, due to reverse voltage appearing on weakest cell, but parallel don't care.

/Estel