Yeah I know its basically got a chunk missing from the middle, but its still classified as a solid beam. And you say 10kg, I can't see a subframe weighing anymore than that either in fairness. I know you have more parts on independant suspension, but I can't see the fact of adding a few extra kg, maybe about 5kg tops with the weight of both wishbones that will really affect the performance of the vehicle, in straight line terms atleast. That 5kg could be the difference in driver weight
When you say you have drove a car with independent rears, its hard to say the affect of handling unless you drive the same car with the two different setups.
I've driven my dads 600bhp skyline, and its a much better drive than the glanza, but its a completly different car, so I can't compare lol.