Probably a tiny bit weaker than full-strength SSJ 2 Gohan. Cell had a bit of an advantage in that Gohan was weakened for their struggle, but Vegeta's intervention near the end helped Gohan finish him off.
At least, that's what I'm assuming. For all I know he could be much stronger or much weaker or something.
So when they abridge the Buu saga will Vegeta remark on how similar Pui Pui is to Cui, in both role and name?
You cannot firmly grasp the true form of Squidward's technique!
Okay, Nap... I mean Team Four Star, what do we have this time ?
Killing Floor ? Well, it's too damn late here for watching this, but this might be intersting...
Err... what? He didn't miss anything. He hit Nappa's entire body with it, and didn't do any damage.
Come to think of it, why did all of King Piccolo's...eggs come out fully grown, but Piccolo Jr. for some reason was a child that reached adulthood in 3 years?
...Man Namekian lifecycles are weird. Wait a second, why did none of King Piccolo's henchmen look like Namekians too?! Heck, for one of them, he just said he wanted a dragon and out came a dragon.
I'm thinking too much about this, aren't I? I mean, it's not like anyone watched Dragonball (not Z).
There's probably some in-universe answer that Saiga somebody can tell you, but the real reason is that Piccolo being an alien rather than a supernatural being was a retcon, and so there wasn't any coherent life cycle at work at the time.
That, and Piccolo Jr. was a very different beast from Piccolo's other minions, both in and out of universe.
I watched Dragon Ball. I actually think I enjoyed it more than Z, to be honest.
Saiga already answered the question about why all of Piccolo Sr.'s kids looked different I think ... let's see:
The guidebooks expand on this by saying any Namekian can choose what their offspring will look like.
That's from the Dragon Ball manga thread. Post 1138.

Maybe, but I doubt it would seriously injure Nappa. Even if it wasn't at full power because of Tenshinhan's injuries, it still did literally no damage. At worst I'd say it was at half power, using Gohan's one-handed Kamehameha as an example (and even then, Goku said that the reduced power was because of his mental state).