What bonuses does my character gain with extreme stats (over 18)?

Read the ability score tables at the end of the manuals.

Thinking more of "hidden" bonuses, such as the regeneration you gain with con 20.

As far as I know, the regeneration from high CON is the only bonus you get from high stats that isn't listed on the record screen, as well as the only one that isn't just a progression of the bonuses you already get (more melee bonuses from STR, etc.)

OK. That's all I wanted to know, Thanks.

i know if you have dex over 20 you can get your ac to -5 with full plate