I love my Thermawrap! It excels as a parka (as part of a clothing SYSTEM or if not extremely cold) and as a sleep system extender.
I generally use it w/my JRB No Sniveller (down quilt). I have been comfortable down to 19* w/it and poly base layers, smartwool socks, and of course a hat (in that case a JRB down hood). Not your exact system, but hopefully it gives some perspective. Without the Thermawrap I wouldn't have made it comfortably at those temps - but I haven't tested this system to its limit without the Thermawrap, so giving you an estimate of how much it extends a sleep system is tough - I would estimate 10deg F.
Note that I didn't needinsulated pants. YMMV.
Let us know what you choose, and how it works out.