This is probably going to be the best muscle car you can buy until the other guys get done with their refreshes, and it improves not only upon itself, but upon it's competitors in nearly every way. A few hundred pounds here or there wasn't going to change that. Quit your whining.
The outgoing model was one of the best track worthy Muscle cars you could buy. It held it's own against an E92 M3. If this model offers even a slight improvement on that, it will be one of the greatest performance bargains ever, and few hundred pounds here or there is not going to change that. Quit your whining.
If you don't like the idea of a heavier mustang, you could always just buy one of those other numerous, sub-3000lb, sub-$40k V8 sports cars...oh wait, no you can't, because that doesn't exist. Quit your whining.
We weren't ever getting a V8 BRZ from ford. If that's what you want, get a BRZ, get an LS1 or whatever V8 you want, and get to work. More work, less whining.
Are you going to buy one? Rent one? Drive one for five minutes? Then how does a trivially heavier mustang impact you exactly? It doesn't, that's how. Quit your whining.