I am on the same mission as you, trying to get my car to handle well and not wonder all over the road.
I have done the UCA drop and the roller spring perch and I didn't notice any substantial improvement.
What did make a huge difference was installing a strut suspension and rack and pinion steering.
The main problem with the Mustang front suspension is that the scrub radius is too big and not enough caster.
The picture below is Fatman's strut, http://www.fatmanfab.com/product/1964-½-1966-ford-mustang-strut-ifs-conversion-kit
However I made my own with BC coilovers for Volvo V70 (I live in Sweden), I kept the lower control arms and strut rods. All I did was to fabricate a bracket to attach the coilovers to the original spindle and bolted it to shock tower using the original holes. I did remove the guide plate spot welded underneath the shock tower. I am running Wilwood brakes so I was able to use the holes for the original brake callipers to secure the bracket. Another advantage is the ease of adjusting ride hight.
This modification decreased the scrub radius from about 80mm to 40mm. I can highly recommend this mod.
Another and easier way of decreasing the scrub radius is to use a wheel with more back spacing.
Although I now can sit and drive with one hand on the steering I am not yet satisfied so this winter I am installing a IRS from a 2015 Mustang.
I am not giving up until my Mustang drives like a modern car.
If you are lowering the UCA make sure you move them back as far as possible to increase caster at the same time.