Well I found that the SC400's that run on the stock exhaust systems mostly will start to get gutless after it reaches a certain high rmp.. Like 5000 or so...
The ECT allows the transmission to downshift faster and carry a higher rpm before the next shift occures... From a stand still start I always found the car to be faster with the ECT function off.... Even now with my after market high flow exhaust and headers.... I think maybe a cam upgrade and head work would change that though...
There is no fix, unless you rework your exhaust including headers, get some high performance cams and head work done... Then you may see your ECT function perform better then when turned off....
That's my take on the ECT....