Doubt E20 would negatively impact power output, unless dyno prooved.
Unlike E85, whether closed loop or wot, E20 have very little difference than Gasohol 95 in terms of power density and AFR, if not for better (slightly leaner would increase power for most cars, until you can't hit certain AFR in 90%++ duty cycle). Also E20 is said have at least (if not better) same 95RON as Gasohol 95. And Gasohol 95 itself is E10, too.
Alas Thailand fuel grades are just s*lly. 91 RON is only 87 AKI if in states, and even worse almost no much cheaper than 95. Should be phased out ages ago. And should introduce RON100 pump gas like in Malaysia!