Factoring in the good solid weld beads introduced, (weld can weigh a good amount sometimes) I would say you shaved about 2.5 lbs at best. If it was indeed 5 Lbs, that is darn good relatively at the front portion of the car.

When I welded in my loop 20 years back or so, IIRC, It was from S&F race cars that I had ordered a chrome-moly tube loop and it was welded to both frame connectors and the floor. My propeller was rather close to the loop as well, but there were no fasteners because it was a tube design. Never did install a rear loop though.