Yeah, it may make boss fighting easier because of the easiness to move in the spaces, but the extra damage (if so, I didn't noticed a big or huge change to how much damage I took) would compensate for it. Besides, you can't use the bat form outside without getting enviromental damage, depending on the region you are (applies for the Ice age and Lava Flats regions).
A suggestion would be to reduce in some way the effectiveness of the spells, wheter it would be in power, casting speed, or something else. Maybe even restricting which spells could be cast in bat form. Another suggestion would be to make the bat larger.