It’s very difficult to say whether someone will be ok in a certain bag. Everyone is slightly different with body temps and sleeping comfort, add to that variables like whether you’re sleeping after a big meal, what type of mat you use etc etc.
You also don’t say what times of year you want it for.
I’ve no experience of that particular model, but used the Marmot Helium (next model up) in conjunction with a lightweight duvet jacket, in full winter conditions on Helvellyn and for bivving on climbing trips in the alps.
The marmot bags are excellent and I think you would be fine with that bag. I’ve always used a combo of a superlight bag and a lightweight duvet jacket (or gillet). It’s your core body that needs to be kept warm and having this combo is more versatile for general camp use, cooking etc.
Added to that, I’ve always wanted to keep my down bags as clean as poss, so generally sleep wearing next days baselayers and a dry clean beanie hat, which keeps the bag clean and adds a layer of warmth. Doing it this way won’t add any extra unnecessary kit.
I’ve also used Mountain Equipment, Rab and PHD, all these companies make good bags. If you’re prepared to spend the money, which it looks like you are, then PHD will probably be the best, they are a specialist in down kit and presumably use the best fill-power down available .
