"Character base Movement Points equal 30 + extra 3 points for every point in agility above 5."
I have 9 agility, 3x4=12, 30+12 =42, plus my tabi boots, which add +31, which equals 73. Which would be my base movement points upon starting combat, which I believe is my 'normal maximum'.
I do not think sprint reveals excess in regards to this normal maximum, I just think it adds to it.
Okay, so I did some testing, freed myself of my entire armor penalty, hit sprint, and attacked someone the maximum amount of times I could in a combat round (with an adrenaline shot as well) to receive this result.
However! Not hitting sprint, attacking him the max amount of times, (which isn't enough to hit the 'cap' but is enough to bring it up to 127) yielded me only 127. So I took an adrenaline shot, and attacked him 3 more times, which should have granted me 154 MP.
However...
It only granted me the aforementioned total. So yes, sprint simply increases your cap as I suspected. This also confirms the feat considers your 'normal maximum' as the MP you have upon starting combat, as I originally stated.