At first glance on the 757 with the window open the ATS will follow to match the speed commanded (no link with FMC) by moving the throttle.
With the window closed the FMC is in charge and issue a target thrust setting. The EEC follows the target by issuing a trim command to his respective HMU, hence no thrust movement unless out of his trim authority.
It does not explain why "it seems" having better perf with window open.