Seems to be a lot of confusion on VFR and IFR requirements. I tell my students, READ THE REGS. If your going to fly an aircraft VFR under part 91 then you must meet the minimum VFR equipment list for part 91. If your going to fly IFR under part 91 then you must have the minimum IFR equipment list, and those instruments/avionics must be properly/ currently certified for IFR. If you add a non TSO piece of avionics, and fly under part 91, not for hire, then it can not replace or effect or interfere with any of the required avionics or systems necessary for the intended flight whether VFR or IFR. Non TSO equipment may be permissible to install and use over and beyond the minimum VFR equipment list, but must be signed off by an appropriate A/P to concur the installation will not interfere or compromise any of the aircraft required systems. These are only the basic guidelines, there may be exceptions to your particular application. If there be some that disagree with my understanding, I invite them to correct me using the appropriate regulations.