The relatively small limit is I hope within the tendency of few methods of increasing your psi pool, the 2 most common ones increase it by about 40 in total (Philosophy and Meditation, the latter being mutually exclusive with Mania) and everything else increases it scarcely.
The difference between a bank and increasing it directly is that if it increases the psi pool directly it also increases the reserves, because the reserves are the psi pool multiplied. Plus it means your extra psi doesn't disappear into thin air because you took the headband off.