Is the improved critical feat just giving you plus 2 to hit whenever you roll a 20 or am I not understanding it. It seems like a poor feat choice if that is the case. I feel like if your rolling a twenty your probably gonna add up to a hit anyways.
The only alternative I can think of is maybe it treats your roll as 2 higher so if you roll a 18 it counts as a critical?
Improved Critical doesn't do anything like that. [Rolling a 20 is an automatic hit, anyway.] It makes it harder to resist the critical effect.
Example: A thunderstrike sonic pistol has a critical effect of "deafen". This means if someone is critically hit by the pistol, they need to make a Fortitude save or be deafened. The feat makes the DC for that save 2 higher than normal. [The sidebar on p.181 describes DCs for critical hit abilities.]
Not all critical effects require saves. Not all weapons have critical effects. The feat is more useful for some characters, and some weapons, than others.