I think that was a deciding factor for me. The cost of the ecoboost was close to a GT. A V6 was significantly less (mine after deals and cashback was $24k off the lot) and at 300hp is already more than anyone needs unless going to the track or 'playing' stoplight racing. For less than a grand, I can get mine faster then an Eco. (TB & long tubes) $2k if I want to be really competitive with some suspension upgrades and a tune, if that Bama tune comes out and shows its worth the money.
The base Ecoboost is about $7k less than a base GT and is about $1.5k more than a base v6. You need to get the 051A pkg on the v6 at $1000 to be comparably equiped to the Ecoboost so you really are only talking $500 difference.
An Ecoboost with a tune, intake and exhaust is in the 12's and will cost you about $1500. You will need to spend at least double that to get a v6 to the 12's.
The serious Ecoboost racers have their Mustangs deep into the 11's without getting into the engine. There is not the support for the 15/16 v6 to build one that goes that quick so you are on your own to build it at big bucks. You would likely spend enough to be in GT350 money.