Did anyone already try replacing plastic parts of CoreXY assembly (mainly tensioners, but might make sense for motor mounts too) with aluminium parts for increased rigidity? I wonder if that would allow for higher acceleration limits.

I am ready to bet large amounts of filament that stiffer tensioner blocks or motor mounts will have no impact at all on the feasible accelerations. The belts and the overall frame structure are the "soft spots" of the mechanical structure, not the PCCF parts. And besides rigidity limitations, there is the maximum force (torque) which the motors can reliably exert without losing steps or blocking, which will also limit the permissible accelerations.