Yep, there is.
Like so. Not the fastest as @Urben best it by 2 seconds, but there is another method that can get about 52-54 seconds. but @fortheseven is taking too long so I guess he's mcfly from now on.
I did get a 5 kill with both CX bombs but can't get SA. Still trying to figure better placement. Give it a go if you can. Let's try and get this SA!!
EDIT: the only thing about the 5 target bomb kill is that you have to wait for 5 explosion kill notifications to pop up THEN wait for the "unnoticed kill" to pop up. But that time, you're standing there waiting time. Killing 3 with the bomb is that fastest I believe cuz I get to the exit as the unnoticed kill shows.
Also, I did a 5 CX bomb kill and got 2 non target kills. Hahahaha. WTF!!!??