I've used the sleep system for several years. I generally don't backpack with the whole system as it's just too big and heavy. In warmer temps (>40) I've backpacked with the heavy bag only and been warm.
I've used both the light and heavy bag together when car camping in temps around 25 degrees. I was a bit uncomfortable. Adding the bivy would give me a few more degrees of comfort, but under 20 degrees I don't think I would be too happy.
Generally I don't think the system is very effective at keeping you comfortable at colder temps. Actually I think that's a feature and not a bug. The military, when spec'ing these out, focused on the cost to keep the soldier alive, rather than toasty warm on a cold winter's night.
Everyone's different though in how warm or cold they sleep. Another variable, since these bags are milsurp, some systems were stored compressed or not cared for correctly, which adversely affects the sleep system's performance. Bottom line, like several others have said, I would test your system out at different temperatures (including the temps you are expecting to use it at), and see how you like it.