No, I think that line did a good job at setting up the Imperial stormtroopers as an intimidating threat. We’d already seen them storm a rebel cruiser in a matter of minutes; then, the consensus is, when they fire on Luke, Han, Leia and Chewie in the Death Star, they’re only looking to scare them off and not kill them, in order to track them back to Yavin. Stormtroopers are threatening enough in the original film, whenever they were such terrible shots in the following films it was simply out of plot necessity that they couldn’t hit the characters.
Besides, Obi-Wan knows what’s at stake in that scene. I think he’s already aware that Luke’s aunt and uncle have been killed. He’s deadly serious.