At what rpm does the I-VTec cut in to increase horsepower
in the V-6? Can you hear it and feel it?

the V6 doesnt have i-VTEC, it just has regular VTEC. it should switch over somewhere between 4000 and 5000 RPM. you can hear it, and possibly feel it too. you have to be at WOT though (wide-open throttle, aka pedal to the floor).