yeah, I wouldn't do it. A couple years ago, I would say go for it. DFW is my home airport, but the gate wait has gotten bad in the past years as DP and the gang have re-banked the hub and packed more flights in.
It would be different if you were connecting to LHR, since there are other flights you could book onto if you don't make the KEF flight.
I've had the same option going DFW-RDU-LHR, with 50 min in RDU. Not taking that chance, since it's the only LHR flight from RDU, and re-booking would be a mess.