The transponders in a Agusta A109 Power have an automatic standby function whereby as soon as you change the sqawkcode it automatically goes to standby mode for a few seconds whilst you finish twiddling... even if its in the transmit position. Sorry can't tell you what make it is...
I've always been told to set to standby manually whilst you twiddle. And when in Standby, you appear as a dot on the radar screen as opposed to a proper 'blip'.
As for mode C - dont a lot of the new TCAS gizmo's rely on that to work out how far above/below someone else you are...?