Hi,
Firstly what exactly are you using the car for? Any track work or is this just a daily?
Secondly, do you mean 5.1 fluid? Because dot 5 is a specialty fluid and can only be used in vehicles that the manufacture specifically specifies to do so, and MPS3 is definitely not one of them. Dot 5.1 fluid is the same Super dot 4 which is rated to about 270-280 degrees. This will be an upgrade if you have just been previously using regular dot 4. It wont help with the bite of the pads, but it will increase the limit to what the brake fluid will boil at.
Regards Matt