Namor wins this fight. Easily.
The key to Superman being so powerful is his huge mix of different powers and the way he can use them in combination with eachother.
Superman in this fight is going to be outmaneuvered, because he has no super-speed. Thus he won't land many hits on Namor.
Also - because you've removed 99% of his invulnerability, he's left much more vulnerable to Namor's hits.
Give him back his powers and he'd whupp ass 10/10 though! 😉