First of all, I've had 4 different Feathered Friends bags and I've loved all of them. Primo.
One of the bags is eVent and it's worked great. However, the full-on waterproofness is only as good as the seam taping, and these bags aren't seam taped. So for sleeping out in the rain, you still need a bivy.
Where I love this bag is snowcaving and igloo camping where the condensation is significant, but not rain. This is where I'd recommend eVent.
My wife has a FF bag with the Nanosphere fabric. This I LOVE. It's more than a normal DWR but breathes quite nicely. It's light, and less expensive than eVent.
For time in your Tarptent, I'd skip the eVent and go with the Nano. For serious condensation, like in a snow shelter, I think eVent (or an eVent bivy) is a brilliant setup.