Idahohunter mentioned one I'll expand on a little. The military sleep system is rated to some ridiculous way below 0 degree number, but that rating is NOT the same as a civilian bag. I've spent many nights using the two bags with goretex bivy as low as about 0 degrees F. Maybe a better insulated pad than the military one would help, but I find it pretty cold under 20. I sleep cold and prefer to be pretty warm, but I would not call that system a good sub 0 solution. In single digits I was comfortable in it if I was wearing a big puffy jacket and pants we called a "happy suit". A tent also helps a lot. With all that said, It is the system I will be using for any hunting I do because it is what I have right now, and I cant afford one that would be significantly better this year.
Lastly, the complete three bag system and stuff sack weighs like 10 pounds and isn't very small. The biggest adavntages are the low cost ($100 to $150 would be a fair price), tough bivy sack, and ok performance.
If you go that route and sleep in sub 20 weather my advice would be...Use a tent, use a good sleeping pad, use all three bags, and wear a warm layer while in the bag. Hot hands packets in the bag helps too, but I don't like carrying stuff like that and having the extra trash to pack out.
Hope that helps. And if you go the quilt route, please do let us know how it works. That may be my next sleeping gear purchase.
And to directly answer what you asked, lol: Yes it would be warm in a tent in 28 degree weather, very adequate for that.