Working under the basis that '1' is considered a normal value:
This means that '1' is the value used to count powerlevels, not the value of a human.
Forget about Lancelot(who was a normal human anyway) - if Assassin, who is established as just a normal human during his life, gets A rank then Satsujinki can get it too.
You are forgetting that Servants are slightly depowered Heroic Spirits, which in turn are only as powerful as they were during their life. Eirei-hood doesn't ADD anything to them, it merely... turns them into a Heroic Spirit.
AGI stat is general speed and dexterity. Assassin has the same speed as Berserker, no matter what excuse you use.(Of course, that wouldn't help him in a fight, but... that's a whole different story).
and the visual novel underlines his weak physical condition (anemia, no sport ect ect.).
This is as wrong as they get. The VN mentions that, while he is of bad health and thus forbidden from club activities, his physical condition is top notch. Otherwise, even with Nanaya out, he wouldn't be able to do a thing.
Just an off thing, but would you mind spell Satsujinki right? It really ticks me off somewhat to see the lack of two letters in the name for some reason.