[Black Lotus]

The Torsen concept address your first sentence. I have a Quaife in one of my cars, the other has the usual limited slip using clutch plates. Neither is perfect, but the Quaife seems to work a bit better.
If you install any sort of limited slip, torque biasing or locker to your car, you may notice more of a tendency to understeer. It's all a matter of shades of gray 'tho, and every car is different. It also may depend on how fast you are driving thru and out of the corner.
The more effectively the inside wheel can transmit the forward driving force of the engine the more of an understeerer you are going to have. But, you might not notice much of a difference.......
Just imagine a rear drive car with an open diff trying to accelerate out of a corner (and doing well enough) until the inside tire starts to spin. The car will hold its line thru the corner well, but won't accelerate very hard because the open diff will simply put the power to the tire with the least traction.
Now imagine the same car with a limited slip. The car can accelerate faster, more load on the rear tires (less on the front).
The inside rear which isn't doing as much of the work of cornering as the overburdened outside tire can now take over much of the job of accelerating. Since the inside tire is inboard of the center of gravity (CG), it will try to yaw the car around so it is trying to point the nose to the outside of the turn. The driver counteracts this with more steering input, and it all shows up as more apparent understeer.
However, if the outside rear tire is well an truly saturated with cornering loads, it may slide the rear end out into more oversteer, and tend to compensate for the understeering (rotating the car around its CG in its yaw axis) effect of the limited slip unit.
Yow!
Like I said--shades of gray.