I'm well aware of what Prima Facie is, it means on the face of, or at first sight, and if you read the transportation code, you'll see that it's still speeding. Here you go:
Sec. 545.352. PRIMA FACIE SPEED LIMITS. (a) A speed in excess of the limits established by Subsection (b) or under another provision of this subchapter is prima facie evidence that the speed is not reasonable and prudent and that the speed is unlawful.
Text of subsec. (b) as amended by Acts 1999, 76th Leg., ch. 663, Sec. 2 and Acts 1999, 76th Leg., ch. 739, Sec. 1 (b) Unless a special hazard exists that requires a slower speed for compliance with Section 545.351(b), the following speeds are lawful
1) 30 miles per hour in an urban district on a street other than an alley and 15 miles per hour in an alley;(2) 70 miles per hour in daytime and 65 miles per hour in nighttime if the vehicle is a passenger car, motorcycle, passenger car or light truck towing a trailer bearing a vessel, as defined by Section 31.003, Parks and Wildlife Code, that is less than 26 feet in length, passenger car or light truck towing a trailer or semitrailer used primarily to transport a motorcycle, or passenger car or light truck towing a trailer or semitrailer designed and used primarily to transport dogs or livestock, on a highway numbered by this state or the United States outside an urban district, including a farm-to-market or ranch-to-market road;
So you'd admit you were going over the limit with that argument, you're still speeding according to the definition as stated in the Transportation Code. You can talk to all the lawyers you want, all they care about it taking your money so you're out 500 bucks and you still think you won. Have at it. It's the same as saying I wasn't doing 80 your honor, I was only doing 75, regardless, you just admitted to speeding.
You can ask for the calibration log all you want, the officer doesn't keep that particular log and no, he doesn't have to log anything nor keep a book for it. The officers word that he checked the calibration prior to and at the end of shift is usually enough and if department policy doesn't dicate it, he won't be found of doing anything wrong.
You or anyone else can do or say anything you want in court, maybe you'll win, maybe you won't, maybe the OP will win or won't. Regardless of Jury selection, who do you think presides over the case? The Judge, and he can still get pissed off and put you in your place in his almighty courtroom (sarcasm). I've seen it time and time again, you asking the question what I was wearing, what was the weather out, what color are my eyes...they don't build up credibility, they don't show anything. The prosecutor will ask you the same kind of question: Mr Smith, what color tie or jeans or shoes were you wearing last year on May 10th.... you really remember? It works both ways and lends no extra credibility other than the fact that you signed the citation, set a court date and feel you weren't wrong. THAT is the basis of the case and that is what will allowed to be argued.
Either way, you got a 50/50 chance, some win, some lose, in the end the cop won't care.