Here is my experience with the whole 2 uppers and a PHB/going to PM3L thing:
With both uppers and a PHB, the ride was awful, and very stiff. The rear felt far too stiff for the car, much more so than when I did not have the PHB. Even on smooth highway the rear would buck around anytime the axle needed to move around. It felt like it wanted to oversteer more than before, but I never was able to autox/track it.
When I removed the drivers side upper, the ride was instantly better. It rode much better than before, no bucking or nonsense. HOWEVER, I have had issues with crazy pinion angles causing the drivers side shock to hit the disc brake hose bracket screw (I have converted to Thunderbird rear discs). No failures, but lots of noise, and it's eroding the shock body (rubs back and forth as the PHB moves the axle), and probably crazy pinion angles on hard acceleration and braking. My diff side bushing is very, very old (possibly 22 year old original... I hope not), so I will be replacing that soon and seeing if it helps, along with a lower profile screw.
Another note. If you go to the PM3L, you will probably want to get some torque arm springs. The rear will be way too soft without them.
My opinion is it's way better, but the annoying pinion issues I've been having make me want to say otherwise. Before I noticed that issue, I was happy as a clam. Sounds like a good number of people use it with no issues though, so it's a crap shoot.
TL, DR : It's a free mod, so it's worth a shot.
1989 Mustang GT: 5.0, 5spd --- Sold. SVO brakes, MM suspension bits, 331ci, X pipe.
2012 Mustang V6PP 6spd MT --- Daily Driver, D Stock auto crosser, maybe track days some time...