I understand why Ford is extending the option but a 4 cylinder - even one with genuine sporting credentials - seems to be working at cross purposes with the Mustang brand, which is thoroughly associated with the V8.
From my humble eye ball observations via the used car world, the Mustang is associated with the being a convertible in automatic. Since 2011 MY, it didn't really matter to buyers of these units if it had a 6 or 8 because either engine propelled the vehicle to speeds greater than they would ever drive at.
The unfortunate reality is that the V8 manuals have a horrible transmission setup primarily because of the remote shifter. Ford has come out and said that they are repositioning the shifter for driver comfort or something to that effect. What they are doing is unofficially acknowledging that the shifter has been a major dud on MYs 2011 thru 2014 and they have a structural fix in hand.
Consequently, the manual transmission in the 2015 V6 and V8 is still a show me story since the same Chinese built Getrag is being used which isn`t the best news. Now the 4 pot turbo might just escape this fate so that in itself would be a major bonus for ordering the 4 banger in manual. Plus the tuner market is going to be all over this motor. I think if your an automatic driver the V6 is fine.