I've lowered and help lower 5 09 to 14 Mustangs and you really need to lower it then make the determination. Before you lower it you should check the pinion/drive shaft angle........then do the same when it's lowered and after the new springs have settled...maybe 250 to 500 miles. As long as there is no vibration you should be ok. I lowered my 14TrakPak 2" on Sportlines and was only off by .5 degrees....on a two piece drive shaft this isn't an issue....on a 1 piece it could be.
If you replace the UCA make sure you replace the carrier bushing with either a GT500 bushing or the BMR #EN-001. If you have more than 7000 miles the stock bushing is probably starting to crack and split..........take a look at it when you have the car in the air.