My T8.1 came out of a MP7 kit, so yes.... they work.
There are a "few" extended mags out there..... the T8 mags need to be modded to work in the T8.1 (upper "lip" needs to be filed down), so T8 Extenders will work. There are also a few T8.1 Extenders out there as well.
Google T8 Extended mags.
-Speed-Wisconsin Current Markers:
Black Nickel T8, T8, T8.1
ICP B5, UMP MK5, RIS MKP