I would probably call that guy in. I would never rat someone out just for speeding, (that would make me the world's biggest hypocrite), someone could pass me on a deserted highway going 150 and I would not report it.
Someone blowing 4-way stops in the city w/o slowing down, however, gets a dime dropped. Just did it the other day, a new Escalade in front of me ran (2) 4-way stops like she did not even see them. (Texting)? One had a woman w/ a stroller about to cross, the pedestrians were left shaking their heads. She could have killed someone. I called 911, got her plate# at her next (forced) stop light and followed her onto the freeway giving updates on her location. (I was getting on anyways). I lost visual contact w/ her and was not going to speed to keep up, hell if I'm getting a ticket out of this and not her.
I figured it was a lost cause because she could have gone any direction on some interchanges/transitions, (62/212/5/494/etc. for MN. guys), but ~30 minutes later my cell phone rang:
"Hello sir, this is Officer Smith from the Eden Prarie police dept. and we have that Escalade pulled-over. What exactly did you see? OK, what do you want us to do?"
He basically gave me the option of coming to their location to make a citizen's arrest, (too far away or I would have been tempted), so I said, "can't you just taze the schit out of her?" (j/k)

He said that he would give her a serious warning and put the stop and warning in the system so that if she gets pulled-over again, the next cop would know about it. (?)
Truly dangerous drivers absolutely deserve to get ratted-out and caught, if that's not what cops are for, I give up.