If you use any shorter springs, then one of two things can happen.
You can have a big gap between the springs, which will make noise and it is a straight MOT/TUV failure.
I wouldn't do that as I don't ever think it is SAFE at all, but different people have different opinion.
You can use a shorter springs with a helper springs, but knowing that the Tein Super Street is a single perch coilovers, you will most likely run out of damper stroke travel and be hitting the bump stop.
This will create an uncomfortable ride and probably kill your damper in no time.
So in short, there really isn't a way to drop your Tein Super Street any lower because the coilovers wasn't design to run low at all.
They probably find out what is the limit and only let you have that much adjustments, any lower will make the ride total rubbish if you ask me.