The INS or IRS meaures the change in groundspeed the same way for both situations.
200 knots North, 180° turn, ending up at 200 knots South in calm air, 200 knots TAS the whole time. Change in velocity: 400 knots South.
In a 20 knot Northerly: 180 knots G/S North, 180° turn, ending up at 220 knots G/S South, but still 200 knots TAS the whole way around. Change in velocity: 400 knots South.
The ground speed matters for navigation, but in terms of how it effects the airspeed of the aircraft, or the forces required to move it about, it is about as important as the speed of a car along a freeway below it, or the speed of an aircraft a mile away.
Same forces involved, same accelerations measured by the INS.
[This message has been edited by Checkboard (edited 10 February 2000).]