Don't recall YJ Deathstroke having much in the way of feats though (didn't like his design there either with the ponytail), but if that's the case with the Canary example (been awhile since I've seen the show), suppose he can...
That was not trying to kill her; come on man. He was pissed, yeah. She said as much. He was just trying to take her to the mat just once to get some of his pride back. Robin laughing at him was not subtle.
When he's fighting seriously, or trying to injure someone, the environment is a victim and his opponent is usually flying through the air. He's taken hits from the Terror Twins, Mongol, etc... You really think Canary putting him on his a*s with technique is enough to warrant him trying to murder her? Or at least put her in the med bay?
As you said Superboy is strong enough to wreck the environment. A pissed off punch from him could very well kill a human being. Maybe I shouldn't have said "trying to" since I doubt he would have meant to but still. There's no way BC should be able to survive a pissed off shot from him. At the very least she would have been very seriously injured
He wasn't trying to take her to the mat. The guy threw 3 full on punches, 2 of which were directly after him getting angry. He didn't try to grab her, he didn't try to trip her up. He was trying to knock her block the F off. The only reason she wasn't sent flying and the environment wasn't destroyed is because she dodged his attacks and put him on his ass again at which point he quit.
So you honestly think he wanted to win so badly that he'd put enough force in his punches to do her serious bodily harm? He's never been that out of control since they found him. The only time he goes that far down the rabbit hole is when he's using a K-Shield.
I think he would have regretted it immediately. Harken back to JLU for a moment. Superman makes a speech while fighting Darkseid and says "I feel like I live in a world made of cardboard, always taking care not to break something, to break someone. Never allowing myself to lose control even for a moment, or someone could die. I know JLU has nothing to do with this show but I feel like that quote is especially important to my point here. Superboy lost control. It wasn't very long and luckily for him he didn't do any damage but for that moment, he lost control.
Superboy had several moments where he gets angry and lashes out. That was a character trait of his. That encounter with BC was very early in the series and he was still a bit of a loose canon. He didn't calm down until much later