More HP is usually always nice, but I think Dodge is about to hit the same wall computer makers did in the 1990s. Back then, the boost from a 286 processor to a 486 yielded significant benefits and folks didn't mind paying $3,000-$4,000 to get those benefits. But before long the computer makers were touting performance increases that were significant from a design perspective, but didn't really yield benefits in how the systems were used, and they reached a point of diminishing return.
Dodge's problem is they are constantly dumping more power into a body design that is close to ten years old, and that is based on a model that only ever had one body design. There are a LOT of Challengers out there, and while a hundred or so more horsepower might be enough to get the interest of a few buyers, many of whom will be the same people Hellcats are marketed to, they aren't going to do much to motivate buyers en masse.
If I were the Mopar Czar, I would get the designers really, really, busy on designing me a 2G Challenger version of the 1971 'Cuda to replace the Challenger. That would renew the interest in the marketplace far better than some more HP in a dated design.
