This is certainly not due to the mps because this sensor has no significant resistance in the system. The mps 400 is basically the same as a 8mm hole....

You should check for air in your pump or something that has clogged due the coolant.

If you remove the mps you can simply look through the hole or blow air through it. There shouldn't be any resistance at this part.

And how long is the mps? 25 cm or 0.25 cm?

Do your math again with a 24.75 cm long pipe with 10 mm ID and in addition a 0.25 cm 8 mm ID pipe.

What is your result?

Not even imagined, that you have to add the restriction of a complete loop and add now the 0.25 cm 8 mm ID. That is not even noticable.

Also imagine every 10 mm fitting for your tubing. Everyone is at least 8 to 9 mm tight in diameter and also more than 10 mm long. So take two fittings and you have the same impact? Would you blame two fittings more in your loop?

Drain the Loop and take the MPS out. Maybe you will get better flow, but I will promise you that it is not caused by the missing MPS.