MLSE, sorry about my ramblings, but I thought I would add a little insight to what the exhaust and radiated heat could be capable of, without knowing more specific details about your vehicle and class rules, I can only assume that by you saying production class, you will be very restricted as to what you can do to your car shape and underpinnings.
So for starters you will have to worry a LOT more about pressure drag than you will skin friction drag. We can minimize that as well as a few other problems with the proper management of the radiated heat from around the engine compartment and the cooling radiators, as in water, transmission and oil coolers.
If you are not running engine oil and transmission coolers, it may be worthwhile to think about adding them so we can add the recovered heat into our system, if not the power you lost from the internal friction of the transmission and the friction and radiated heat from the engine being transferred to the oil, will be lost, so no matter how negligible, it is a source of power we can use to our advantage.
If we do not recover this source of energy and put it to use, your competitor will.
I’m sure records are set and broken by fractions of seconds and tenths of miles per hour, look at all the small details and in the end they will add up to be a contributing factor to the overall package.
I have lost races by .002 seconds before.
If rules permit we can build a type of duct system around the engine to recover the radiated heat and direct it to the underbody, increasing engine cooling and improve aero forces.
I will assume also that you will be very limited to what you can do to the actual vehicle underbody. We can’t install a full underbody tray, but can we use air flow directors and vortex generators. Are air dams, splitters, diffusers and spoilers permitted.
I wish I had a rule book to review, is there one on line I can look at.
Without knowing more about your particular class and the type of car you have I can only speculate.
Give me a LOT more details about what you have and what you wish to achieve.