I have acquired an early AL-800 SR# 56 and I wanted to check and see if any one knows of any important updates (circuit updates over the years) I should make to the amp before I start with the second tube modification.
No. There are no significant mods.
I regularly run my AL800H at 1500 watts all modes, and it will easily do over 2300 watts on SSB or CW. Except for RTTY or sustained carrier operation, it will be tube limited. You are correct in your thinking.
I ran a single tube AL-800 amp on 6 meters at 1200 watts, and converted it to a pair. It can do near 2500 watts PEP and over 2000 CW now. I would NOT run it at RTTY or FM any higher than a single tube would do, because it is transformer temperature and tank heating limited on RTTY or FM. It is NOT transformer limited on SSB or CW.