There are a lot of examples of ordinary humans beeing enhanced in some way in the W40k books.
- Fabius Bile likes to experiment and often makes genetically enhanced humans
- Failed aspirants for space marines are enhanced
- Inquisition lords are enhanced (mostly coz they have the money and the knowhow)
- Skitarii are enhenced
- Officio Assasinorum (Imperial Assassins) are enhanced
So yes there are many, many examples in the books.
Most of them arent enhanced to be better than a Space Marine tho. The process to have the same organs as a space marine takes years and has to be started while the subject is still a young adolescent. Otherwise the organs dont bond and there is a huge risk of the subject dieing from the prosess.
However implanting rage stimulants, pain supressants, muscle growth, anti ageing (juvenant) treatmants are all accessible to the higher tier of Imperial society so you regulary meet Planetary lords, Warmasters, Generals, nobility and the like with some combination of enhancements coz they have the money and access to the best care in the Imperium.
The OP put a link of Rex in his question detailing his early history, acolades etc.
As Rex grew to adolescence, he was subjected to genetic enhancement by the Imperium's finest chirurgeons and gene-adepts, similar to that worked upon the superhuman bodies of the Adeptus Astartes. His physical body grew to massive proportions until Rex towered eight feet tall, with a heavily muscled frame and superhuman physical condition.
So that in itself answers if Rex's enhancements are similar to Space Marine enhancements. There are Space marines that are described as beeing 8 feet tall (Pasanius).
Another such individual is Fabius Bile. He started as human, became a space marine, then the apothecary. Then the horus heresy happened. He is obsessed with improving on his body and also obsessed with making beeings better than Space marines.
The lore is full of characters that rival or even surpass Rex and his abilities. The Imperium of men has million of worlds and uncounted subjects.
Edit 2: Just to stop this theory-crafting that Space marine is better/stronger than an assassin. Clearly the below stats from the codex Space Wolves and the assassin codex show that one on one an Assassin will eat any Space Marine for breakfast. Better at waapons, better initiative, more attacks. So they will come fast and hard and they will end you.
This is Ulrik The Slayer: an officer and highly trained Space Marine
This is an entry for "Ordinary or line troops"
This is an entry for a Vindicare Assassin. But the stats are the same for all the temples.