I believe almost universally Battletech works on whatever the 'current' set of circumstances are. Can you describe a little more why you feel it should still be at the 8MP level?
What page in TO are you referencing?
Pg 55 TO under High/Low Gravity
Our thinking for keeping the PSR for damage check at the 8 Running MPs was the leg is designed to carry the 'Mechs weight Running at full speed. Even with the 'Mech dragging one leg behind it the functional leg is still taking less dynamic load moving at 50kph then it would with 2 good legs moving at 80kph. And the High/Low Gravity damage feels like it is coming from exceeding design stress limits more then just going "too fast"
The people thinking the check should be at the new lowered Running MPs is basically what you said, "Battletech usually works on whatever the 'current' set of circumstances are."