How this could happen
This is definitely possible. A simple way for a parasite to accomplish most of these changes (aside from telekinesis, which I don't think has any biologically plausible explanation) by modifying the DNA of the host, likely by introducing some retrovirus into the bloodstream to selectively modify the DNA of specific types of stem cells. Especially if the parasite infects children, achieving peak physical state is just a matter of growing larger and producing more muscles.
Even in adults, though, those changes are possible: we stop growing because we're genetically programmed to stop. There's no biological reason why a virus couldn't switch muscle production back on and increase mass to some point beyond what an ordinary human would reach.
Eye color can also change, especially if you want the eyes to become darker. Eye coloration is based predominantly on the amount of melanin in the eyes. Babies are born with pale eyes for this reason. Just as a virus could change your DNA to produce more muscle, a virus could alter the DNA of your eyes to produce more or less pigment. In an adult, if the pigment already exists in the eye, lightening would be a much slower process than darkening, as the existing pigment would take time to naturally degrade, even if the production of melanin suddenly stopped.
How the parasite could attach
The parasite could attach itself just about anywhere. Primarily, it just needs to interface with the circulatory system in order to regulate viral levels in its host's bloodstream. Popular locations for real world parasites would be either in the digestive tract, or under the skin. Parasites are capable of moving about in the body, to some extent, so they can end up just about anywhere. However, tunneling through living tissue is generally bad for the host, so this is probably best kept to a minimum if your goal if physical enhancement.
If your hosts are intentionally infecting themselves over a long period of time, it's likely that they'll select for less painful and less harmful parasite attachments. Subcutaneous parasites might be the best option: they won't cause as many skin lesions if they're under it, while still being removable if they become problematic.
In terms of downside, the primary question is: why aren't all humans gigantic and muscular? The main reason is probably food. Muscles take energy, and gigantic muscular people take a lot more of it than normal sized people. Neanderthals were substantially more powerful than modern humans, but had to eat four times as much food to sustain that. Parasitized humans would have the same issue: they'd do fine when conditions were good, but die off far more easily in lean years.
They'd also be far more susceptible to cancer. Cancer and growth are two sides of the same coin: if you modify the genome to create more growth, you increase the chance that something goes wrong. This is true even without parasites: large dogs, for example, are more susceptible to cancer than small ones. This issue isn't insurmountable, however. Whales and elephants, for example, have very low rates of cancer formation. Largely, this is due to genetics: both whales and elephants have multiple copies of genes found in humans which prevent cancer formation.
If the parasites have evolved naturally, they're unlikely to make this sort of a modification to their host's genome. Their evolutionary strategy, more or less, is to make hosts that can be very successful in the short term due to their increased physical prowess at the cost of long-term risk due to factors like starvation. It could be selected for in the long term, though.