I see one HUGE flaw in the Heatsink and shrink wrap design.
If you did not know Air is a VERY GOOD insulator. Air is now trapped between the cool fins.
This means heat is trapped between the cooling fins and can not escape.
The heat dissapation of the orginal ESC might even be better without the Heatsink and shrink wrap.
Also being stuck in the Styro fuse without airflow it just compounds the heat retained.
I would remove the shrink wrap in the cooling fin area.