They would look like this, apparently:
This Guy's Eaten Nothing but Raw Meat for Five Years
But onto the meat of your question:
Long, strong legs for running and chasing (or even the ability to alternate between being on 2 legs and all fours when chasing prey),
We've already got excellent legs for running and chasing. Sure, we can't outrun many prey species, but it turns out we're still pretty good at endurance hunting.
possibly sharpened finger nails/claws, a re-emergence of giant primate canine teeth (this time with sharpness)
Seems unnecessary. Even now, we can see that other primate species like chimpanzees, who like a bit of fresh meat from time to time, can hunt and kill monkeys without the need for claws or massive sharp pointy teeth.
A few relevant videos:
a shortened digestive tract (a common trait of carnivores)
This might actually be plausible, but consider that an obligate carnivore might have serious problems coexisting with an intelligent omnivorous species that might have access to much more food sources in difficult times and would perceive their cousins as a threat.
a skeletal-muscular system mostly consisting of fast-twitch muscle fibres (high power, low endurance, similar to a feline species)
This runs directly counter to our already practical high-endurance, low-speed setup which turns out to work OK.
possibly a different setup for mounting muscles on our jaws (something more akin to a carnivore like a Tiger, an offshoot of the sagittal crest paranthropus had).
Maybe. It isn't obviously necessary... remember that animals like tigers use their jaws as a weapon and so need quite radically different neck and head musculature. Surely your proposed hominids would be using different techniques? (see also: chimps hunting monkeys).
Would this species still even be capable of evolving into a sentient homonid race like us?
We don't really have enough to go on to answer that question. I would suggest, though, that such a species would be unlikely to coexist with Homo Sapiens or its forebears, and we know what happened to the other flavours of hominid that used to live on this planet, don't we?
Consider that one can be a carnivore without being a hands-on predator. A sufficiently clever and organised scavenger species could conceivably drive off dangerous predators and steal their kill. Humans have certainly done this in the past. The ability to craft weapons would certainly help, but even the ability to throw rocks effectively is a trick that is vanishingly unusual in the animal kingdom.
See also tricks like this, where one can use brains to kill large, fast and dangerous (but above all, tasty) megafauna without having to evolve into a tiger.
Acquiring sufficient calories this way might be tricky, but evolution isn't required to act in the long-term best interests of any species, and a previously omnivorous species evolving into an obligate carnivore due to some unfortunate mutation, population bottleneck and genetic drift could conceivably distribute the deleterious mutation through an entire population forcing them to become meat-eaters even if they weren't previously.
This could explain your species, though it would come with almost none of the big-bad-predatory-physique changes that you seem to want. Sorry about that.
For a different take though, have a search for Peter Watts' take on vampires. He has some interesting ideas there which may be of interest to you.
Irritatingly, I can't find a succint summary of his ideas, but there is a youtube version of a fictional lecture he produced on the subject, if you were bored.
His vampires were hominids that preyed on other hominids (for reasons that are explained in universe that I won't repeat here)... they were very strong, certainly, but primarily they were very intelligent, as would be required for hunting intelligent and social prey. They were also hibernatory, making it easier to lie low between meals, and avoid stirring up too much trouble in local human societies.
Its not quite what you're after, but interesting nonetheless.