I think you need to define "winter," and also decide whether you want it work in "winter" unsupported by additional clothing, etc.

The 30-degree rating is accurate; I've slept in it, with the hood snugged down, at 25 degrees with only a pair of lightweight long johns, and been plenty warm. Use heavy weight longjohns and maybe a down vest, I'm thinking I could go 20. (If I were going to try that regularly, I'd have an insulated jacket and pants - like the WM Flash hooded jacket and pants - in reserve. The Megalite is roomy enough that a "normal" build will have enough room to wear them inside the bag to bolster its rating.)

If you're looking for a winter bag for temperatures lower than 20 or 25. look at the Alpinlite instead. It's rated to 20, but I have spent a zero-degree night in it, with midweight longjohns and the Flash jacket, pants, and booties. I slept very toasty in that combination; I don't plan on it as a regular practice - it was one of those nights when the forecast was for twenty.

The other key, as you already know, is the pad you use. I spent a very chilly night in my Megalite on a Big Agnes Insulated Air Core pad; the temperature never got below 35, but the pad wasn't up to the challenge (the rating on the pad was far too optimistic.) Siince then, I've slept in the Megalite on a NeoAir All-Season and a Big Agnes Q-Core (different nights, not stacked up) on 20 degree nights, and been toasty both times.