Thats true to a degree, front wheel drive cars grip corners primarly with their 2 front tires, in a turn at the limit of traction power going to the outer wheel could cause it to slip causing more understeer than less. With an open diff that power would have gone to the inside tire which isn't being utilized as much as the outside tire. In other words the inside tire can afford to have more power routed to it.

I would worry about how quickly the LSD starts to send power to the outer tires if I was worried about understeer.

A good example is the Type R. It has alot of other modifications to limit its understeering tendencies. If the GSR had a LSD it would understeer alot more than it does now. You could balance it out by adding stiffer sway bars and strut tower bars in the rear just as Acura did with the R. This is why alot of people "upgrade" to R parts to increase handling.