I'm currently running a PC3 on my Heritage and it runs great. Will I notice any significant difference in engine performance if I upgrade to the 5?

 

No.

In anything but a really angry modified engine IMHO the III does it well. In fact in street riding the V gives up a nice feature...map comparison...which can be overcome by creating spreadsheets outside the software. Too bad. On a day to day ride the only real upgrade is the V does not need the 9 volt battery connection as it powers through the USB so hookup is neater and faster. Yes, it does expand on III features and details...but, no is still my answer

 

They are for different year bikes. The higher numbers are most likely due to the bigger engine.

 

If you have fuel injection and 02 sensors you can add the autotune to the PCV.