This program should definitely get you off that plateau! I am currently on week 4 of Meso 1 and I have already seen amazing results from STS. It really does help to do the 1RM testing and have it all laid out for you...especially in Meso 1, because the rests are so short. I print out my worksheet ahead of time and get all my weights ready before I start so I don't have to pause the workout (unless I really NEED a longer break-heh!) You may also find from the 1RM testing that you will need to buy heavier weights than you are currently using.
It seems like people are all over the place on how they do their 1RM testing. I did mine in a couple of days focusing on the 5 star exercises first, then 4 star, and 3 star. I just alternated body parts and took rests as needed. I even did laundry and housework and fed my kids while I was testing, so it took me a few hours each day to get through it. It was kind of difficult to accurately test my 1RM for some of the leg exercises, like squats, because I didn't have a spotter, so I did do some guesstimating. I think it is better to alternate muscle groups while doing 1RM testing, or your muscles may get too fatigued after a while, skewing results.
I think you will really love STS-Have Fun!
Jules