I work for a city agency inside Orange County. With that said, do realize I am not assigned to traffic. I am routine patrol working towards K9unit so essentially I am not required to do any traffic stops. I personally feel night shift officers are a bit more forgiving with speed. My department will not stop you at night for 10 over, but some will stop you for 15. However, there are traffic units that work for Orange County and Orlando who may stop you for 10 over on state roads.
I would say you should get away with 70 over on I4, but 75....I can't ethically tell you, you wont be stopped for that. Nor would I want to tell you that is ok and then get stopped. You are right with the way traffic is on many of our major roadways. As I am sure you have noticed as well it seems we have people dying on our roadways weekly and often daily. Many of this is due to speed, I can sit on i4 and within every 15 minutes I will get someone at 90+ on i4. I will tell you I work a small stretch of I4 on some nights, and I PERSONALLY do not stop anyone until 85, thats my limit. The state troopers differ, and some will stop you at 75....not quite as often though.
Orange County S.O. typically wont stop for 10 over either unless they are specifically assigned to traffic enforcement. I know this doesnt really answer your question...it is just hard to say...each officer is different. Most of us who are seasoned and have been doing the job for a longer time wont waste our time for someone who is doing 10 over and causing no one else harm. The new young officers though...hahah, never know what they are gonna do or gonna stop.