Actually, Mike beat Raph in sparring twice (issue 2 and the Raphael special). If you will recall issue 2, the fight ended with Mike tossing Raphael into a bookcase with a rolling technique. In the Raphael special, Mike was dodging Raph's attacks and connecting with his own almost without effort (a fact which greatly enraged the proud Raphael). That fight also ended with Mike tossing Raphael to the side.
Much of this information (regarding the turtle's attributes) is gleaned from the TMNT rpg by Palladium which was written around the time that issue 4 came out. Eastman and Laird worked very closely with it and contributed all the artwork for it. The character profiles written therein are pretty much regarded as the 'official' profiles for the mirage tmnt. In them, Mike has a Physical Prowess of 24, Leo has a 20, Raph has an 18 (i believe), and Don has like a 16.
In the character profiles for the new, mirage-based animated series, Mike is also described as having a phenomenal level of agility (compared to the others). The other character profiles for the new cartoon seem to mach the comics characters precisely.
Mike only gets hurt more in fanfictions

Raph tends to get hurt more in the comics due to his hotheadedness.