The cooling additives are simply surfactants that reduce bubble formation in the coolant where it contacts the hot iron surfaces in the water jackets, to minimize nucleate boiling. Fresh anti-freeze already contains the same surfactant in the coolant's additive package, but if the coolant is over two or three years old, the additive package has weakened, and won't be as effective.
The cooling additives are designed to be used with WATER as the coolant (like under track conditions, where anti-freeze is banned), to add the surfactant to the water; if you have relatively fresh anti-freeze, you already have surfactants in your coolant.
Check the actual temperature at your thermostat housing with an I.R. gun vs. the temp indication on your gauge at the same time so you know what the gauge is really telling you; Corvette temperature gauges are famous for showing higher-than-actual coolant temperatures.
If you really have a cooling problem, you won't find the solution in a bottle; check out next month's issue of "Corvette Enthusiast" magazine for a comprehensive technical article on understanding your Corvette's cooling system.
I'd like to add a couple of thoughts to this.
First, it is not true that all antifreeze products have the "same" surfactants in them as do addotove products like "Red Line Watter Wetter". Some do bit some do not. In addition, the antifreeze which may have the same surfactants may have them in lesser strengths or proportions.
Second, the part about nucleate boiling is correct, ie: if such activity is present in the system, then cooling additives which are surfactants (be they in a pour-in additive or are blended into the antifreeze) will be effective in reducing or eliminating nucleate boiling. What one must remember is not all cooling systems have problems with nucleate boiling and you can have a car that runs hot without nucleate boiling being present. In that case, you can add surfactants (either with an additive or with antifreeze that includes them) until the cows come home, but you will see no practical change in ECT.
Third, the blanket statement that bottled cooling system additives composed of surfactants are intended to be used only with straight water coolant is not correct. There are a number of these on the market. Some are designed for water-only coolant but others can be used with either straight water or water/antifreeze mixes. To determine whether or not a specific brand of additive is compatible with water/antifreeze mixes, read the product's directions or consult its maker.
As for the question on the 81, running 200-240 degrees, my guess is that using a coolant additive is not going to change that much, though it would not hurt to try it. I'd also add that the temperature range cited could be considered normal under some circumstances. It would be helpful if some data was available about under what operating conditions and weather conditions that engine coolant temperature range was observed
That said, initially, I'd look in areas other than the coolant itself for the cause of the high ECT.
First---as suggested above, make sure the instrument panel temperature gauge is accurate by using an IR "temp gun" or a known-good gauge.
Second---make sure all parts of the cooling system are functioning properly. This includes checking to see that the front of the radiator is clear of debris, the front air dam is in place and functional and that the sponge rubber seals which prevent cooling air flow from bypassing the radiator around the core support are in place.
Consult the factory service manual for an 81 to gain information about the cooling system. Do not rely on aftermarket service data or Internet rumors.
