Mid and low don't take exactly the same amount of time. I don't know where you're getting these stats but they're definitely not right, no one would use the suros if it wasn't the best auto rifle buddy
Low impact ARs fire at 900 rpm(0.066s per shot) while mid impact ARs fire at 600 rpm(0.1s per shot).
Mid impact ARs take 10 shots to kill(1.00s)
Low impact ARs take 15 shots to kill(1.00s)
The reason people like to use SUROS is because of reputation. It's just like Felwinters. Matador is objectively better than Felwinter's, but people like to choose Felwinter's anyway due to its rep.