1, cooling is important, normal alternators are not designed for putting out the amps for long periods.
2, I would look at 120-170amp alternator
3, Not worth the money on a low amp alternator - they work by increasing the voltage on older alternators. It will also make your alternator work harder (if your alternator doesn't have a decent voltage output already), but your alternator already struggles putting out the amps anyhow!
3!, A large bank or small bank you still have to put in what you took out and with a larger alternator a large bank will be able to take more amps in the bulk stage.
If you use little you may be already charging okay as the symptons above where the amps drop down to 5amps sounds like your batteries are nearly full, where are you getting the 80% from - that's probably the thing thats off.