I would start working on strengthening all the rotator cuff muscles, not just the subscap. Also, work on the serratus ant and lower traps could help as well.
I frequently get impingements and I have found one exercise that helps me a lot. From Doggcrapp:
With a large towel or broomstick I want you to hold it with straight arms for the entire time of what i describe in the following movement--a large "rolled up like a rope" beach towel works good but honestly a longer broomstick (without the bristles) works best in my opinion.
Start out with it with a really wide grip (with straight arms) in front of you (on your quads) and with straight arms bring it up and overhead and then down and back to the middle of your back--STRAIGHT ARMS ALL THE WAY--this is going to be very difficult and hard the first couple times out and then will be "old hat" with time----and its going to be painful in a stretching pump kind of way---
i want 50 reps each time you do this--one repetition is from in front of your face (all with straight arms) to up overhead and back, and then down all the way to the middle of your back and then back up overhead to in front of your face again (again all with straight arms)--the important part of the movement is the area overhead that is really tight--do all of this carefully/slowly---dont just whip it over and back---
if your hand is slipping off the broomstick even with the widest grip, or you cant bring your arms over straight and the start bending on you, you have some serious shoulder inflexibility and need to work this hard and get up to speed (or you could just need a longer broomstick too)--again do all of these revolutions controlled and carefully--push into the stretch as you go along toward the 50 revolutions, your chest will be pushing outward and your shoulders rolling back--
your shoulders are going to blow up with so much blood its going to be incredibly painfull pumpwise--Do this once a day at nite as many times a week as you can---sometimes I have people do it every single day---but every time you do it try to move your grip inward (thats the key)----its going to be very hard to do but try your best to move your grip inward for the next 2-4 weeks and your range of motion with shoulders will increase dramatically and any impingement and the majority of other problems should be gone in 2 weeks--
also try to move your grip in as you are doing the 50 revolutions--start off with a stretching but relatively easy 10 to warm up some, then try to move your grip in even by a centimeter if you can for the next 20 revolutions and then at 30 try to move the grip in another centimeter--really try to push what you can do stretchwise once your warmed up here--trust me this sounds easy but your going to be muttering "fuck you dante" after you get to your 25th revolution--
Ive cured too many shoulder problems with this simple movement now its pretty ridiculous, and this and a menthol rub applied liberally daily and before sleep has cured alot of shoulder/bicepital tendonitis in trainees ---Heres a pic attached to this post so you can get an idea (thanks to a trainee of mine who cured his shoulders with this)--but remember the broomstick goes overhead and all the way back to the middle of the back (he just drew the start of the movement when you begin)