Yes if you get the genetic engineering right. You'd have to do more than just change the genes for height in order to overcome the problems with someone whose body's otherwise designed to be around 6 foot growing to 8 foot. I wouldn't go into details about this though, just saying that people have been genetically engineered to be taller, stronger and more heavily built would be sufficient.
Not sure what you mean by implant enhancements though. Genetic engineering is sufficient to make people bigger.
You could also selectively breed humans to be bigger, as long as you're careful to avoid genetic illness caused by inbreeding (same as with dogs, horses etc) - it would take more than 100 years to do this. That's only about 4-5 generations in humans. Dogs can breed at two years old, so 100 years is around 50 generations of dogs, so changes from selective breeding happen much faster. (Although if the world isn't concerned about ethics at all, getting humans to breed at age 13/14 would give you more generations in a shorter time.) Or you could combine selective breeding with genetic engineering to speed things up a bit.
Note that all of these things are unethical on a massive scale. As far as plausibility goes for fiction, your world's political system would have to be sufficiently bad to explain why such things are allowed to go on. There is no way I'd find it plausible that a democratic society with human rights legislation would allow any of these things to happen. But if your world is dystopian, then yep, it's a good way to get your 8 foot tall humans and show the society's callous disregard for ethics at the same time.
