Urea is a good thing compared to DPF, It is a fill once every 5000k or something like that. I was gonna buy 2010 F-350 Lariat with al the deletes, and goodies. I took it out for a good highway run and it was showing 11L/100 which is very good. The 6.4 was reliable, just a pig on fuel without the DPF, exhaust and programmer. This truck was a ROCKET!
If you are worried about mileage, you shouldn't be shelling out 60G's plus on a truck then.
JUSSSAYIN'\
How so? Considering DEF does not eliminate the need or use of the DPF, it's only purpose is to reduce it.