Yes many have used JB4 piggyback. The JB4 is ok but best on custom map from and good experienced VT tuner and using the fuel wire with a good ecu base tune from and good experienced VT tuner. Will never be better than ecu tune from a good experienced VT tuner but does have end user adjustability.
For the g4fj?
I did a lot of logging with a race chip looking for some other differences since it has access to the IAT and saw nothing. If I remember right the uncle chip had adjustability that made me think it was messing with the IAT signal but I've never gotten my hands on one.
Jb4, not sure.