Ok to start of the counter arguement was created that if there was a V6 and a 300 horsepower model for under 20k and only seperated by a 1000 bucks(which will be here for referred to as the LX I totally forgot about the LX Foxes of the 80s which adds some credibility to the statement as it would give proof of Ford doing such a thing in the past) that it would hurt V6 sales. Well I don't think it would impact V6 sales much due to the fact a lot of people who would buy the V6 Stang aren't intrested in power and would want to save the extra grand and the extra money on insurance. Now where this might steal a few sales from the V6 market is the guys who want the V8 power but can't afford a GT and still want to buy a Stang now think about this. In the past their only option was to go to the V6 and try to mod it up because at least they are still driving a Mustang. Now you'd be able to please those guys and make a little extra cash to boot as I doubt(again purely speculative backed up with a little common sense) it costs Ford 1k per engine to produce another 4.6 V8 to go into LX models.
Now the counter arguement sways to it would hurt GT sales.
And I would argue not really especially when you consider the vast differences in interior,creature comforts, and exterior would be so different a lot of guys would gladly pay an extra 5k for that upper level look and finish. Number one example of this...the Ford Crown Victoria and the Mercury Grand Marquis. I've had the pleasure of riding and driving both these cars from the same generation quite a lot(my mom had a 00 Crown Vic and traded it in on an 01 Marquis)the sticker prices on the cars where different but in the Grand Marquis she got a much nice interior but the performance was the same. There will still be people who will pay a premium for a superior interior and an image. Personally I'm not one of them.
Where would the market be for a 29k special edition Stang be? It would still be there. If Ford makes the Mach(designed for insane straight line speed) or the Boss (for great straight line speed and even better handling) it would still sale. Both those cars would likely have substantially more differences between the GT and the special edition. Likely a bigger engine better gearing better
suspension IRS etc...basically the special editions would be tailored to a market and that market would eat them up and the LX and to be honest the GT would have little to do with those sales.
And finally it would cause the Cobra to move down in price. Well no it wouldn't considering Ford has already said they are going to aim the 06 Cobra at the Corvette and that they are moving the car upscale in the market. From my knowledge Ford never said they are positioning the pricing for the Cobra based on the rest of the Mustang line. While I'm sure they'll keep it "affordable". I doubt that a potential LX would cause the Cobra to some how drop in price by 5k bucks. Two totally different class of people would be buying the cars. The LX buyer would likely be younger(late teens 20s-early 30s),not as wealthy as a potential Cobra owner,would want to likely mod the hell out of the thing, and would beat on it relentlessly. The likely upscale Vette fighting Cobra owner would likely be much older(40s-50s), money to burn(as the car would likely be a good 4k-5k over the sticker of the current Cobra at least),not as prone to mod it highly or beat on it like an LX owner. Two totally different animals so the pricing on one wouldn't really affect the pricing on another especially if they are seperated by two other model levels.
Man that was a long post and again....I'm just playing devils advocate(I like a good debate).