I'm arguing that the "5 minute response" on-call should be at regular or OT rates. If your on-call rotation is like a 1 or 2 hour response time, then I could see it being less, but the problem is that I've been at a company where the on-call was previously "whenever you get around to it" and later they changed it to "within 30 minutes" and I was not compensated any further even though it killed my life anytime I was on-call.

Why I believe it should be at the full-rate: because I don't trust the company culture to stay the same over my tenure there. My expectations for a "shit company" have to be the same as my expectations for a "good company", because a good one can turn to shit quickly.