Yes, mine uses the T-15 and currently on my 9th year. It produces about 1.45 lbs of chlorine per 24 hr period at 100%. So in 12 hrs at 70%, that would be about 1/2 lbs per day or about 3 ppm per day added to the pool which would be on the high side. So either your cell is indeed failing or you have something else in the pool that is consuming a lot of chlorine.

Have you done an overnight free chlorine loss test? You should not lose much chlorine overnight and if you do, then it could be something growing in the pool.