A star with absolute magnitude of -7 is big and powerful, It would be among the most powerful stars in the galaxy.
The star would be near the end of its life, so comparing with other aging stars:
Aldebaran is slightly more massive than the sun. It is a red giant (or orange giant according to some sources) It has an absolute magnitude of about -0.6. Aldebaran is too small to undergo a supernova.
Betelgeuse is much bigger. Estimates of its mass put it at about 11 solar masses, but it could be bigger. It has a magnitude of about -6 (it is variable and there is some uncertainty on its distance).
Eta Carinae is one of the most powerful stars in this part of the galaxy. It is also variable, but has an absolute magnitude of -8—-9, and mass of 100-200 suns
AG Carinae is a LBV with a mass of 50 suns and a minimum absolute magnitude of about -8, but becoming much brighter during outbursts. (as bright as -11)
So an evolved star with a magnitude of -7 could be supposed to be between the LBV stars and Betelgeuse, with a mass of around 15-20 suns. Stars do get very unstable this late in life, and there could be a great deal of uncertainty.
Others here know more about supernovae mechanics, and may be able to explain any discrepancy between the supposed stellar mass, and the mass of the ejecta.