Not sure what you think you're getting at Soro, but the entire ability stacks with Improved Critical, yes.
And that is what I brought up. I was observing the fact Iron Power its self stacks with Improved Critical followed by a build with a superior critical range.
A 18~20 crit range is three numbers wide. 18, 19, & 20.
Exotic Weapon Master adds +2 to that for a five number total: 16~20 or 16, 17, 18, 19, 20.
Improved Critical (x2) + Iron Power (also x2 at 4th level) add together becoming an x3 multiplier.
5x3 is 15: 6~20 or 6, 7, 8, 9, 10
, 11, 12, 13, 14, 15
, 16, 17, 18, 19, 20. (my post said 5, I was off by one).
Comparatively, more Dispater focused build has a 9~20 range.