the 6.3 0-60 and 14.9 1/4 comes from i think it was motortrend...it was one of the mags that puts the yearly tests in the back...you know what i mean? the huge list of everything they have tested that year. somebody had it up on their website for a while, i can't remember who. im not positive if it's true, but it might be. supposedly it was a test with the pony package. the better tires sure do help alot from my experience, and i guess it is possible, but not very likely that they could drop to 6.3.
http://www.modernracer.com/fordmustangv6.html
those are the best times i have seen myself and they seem pretty accurate, but then again that is with a stock, base model 6. i strongly beleive that with the pirelli's you could drop a tenth in the quarter cause they definately hook up alot bette rthan the stock traction t/a's.