A creature with Improved Evasion takes full damage after rolling a critical failure on a Reflex save against a damaging effect.
The important part here is "When you roll a failure/critical failure", only the result you rolled matters, not what it was turned into by class abilities, spells, or whatever.
No ability will ever change your degree of success by more than one step, this was added on the errata:
Changes to the Greater Juggernaut, Greater Resolve, Improved Evasion, and Third Path to Perfection class features
All three of these abilities grant a two-tier benefit on a failed saving throw of the specified type, but (as always) no ability will ever change your degree of success by more than one step. To clarify, we’re making the following clarification to all three abilities. Change the beginning of the last sentence from “When you fail” a given saving throw to “When you roll a failure on” a giving saving throw.
The different levels of damage depending on your level of evasion for basic reflex saving throws would be:
Save Result |
No Evasion |
Evasion |
Improved Evasion |
Critical Success |
No Damage |
No Damage |
No Damage |
Success |
Half Damage |
No Damage |
No Damage |
Failure |
Full Damage |
Full Damage |
Half Damage |
Critical Failure |
Double Damage |
Double Damage |
Full Damage |