Oops, I must have posted too early. Haha.
In a sense, all points made here are somewhat correct.
High temerature effect on membrane:
The most common RO membrane, DOW Filmtec membrane, is made of a thin polyamide film. Polyamide is another name for nylon. It simply means that the membrane is a long, repeating chain of amide molecules. An amide is nearly the same group as amines (found in amino acids, proteins) and thus are sensitive to heat. As an example, consider cooking meats in your oven. As you heat up your meat, the proteins in the meat begin to break down (meat and bacterial proteins alike). Since all proteins are made of amino acids, you are doing nothing more than denaturing proteins and breaking some of the bonds to the amine groups. On average, the lowest recommended safe temperature for chicken is ~160 F. This is recommended because the proteins do not break down for sufficient microbial die-off until around 160 F.
That being said, we may apply this same knowledge to the RO membrane. The polyamide film will begin to break down and weaken at high temperatures (>150 F). Since most weater heaters are under 140 F, the threshold for burns on human skin, the membrane will most likely not tear immediately. However, using hot water repeatedly would ensure membrane failure. The length of time to failure certainly depends on the temperature of your water heater. DOW lists a maximum operating temperature of 113 F (as shown in the link above).
High Temperature effect on ion concentration:
In general, as the temperature of the water increases, the concentration of dissolved solids increases. The increase in temperature provides the necessary energy to dissolve the metals. Although, this increase in concentration is only a very slight one for the range of temperature increase in this situation. For the most part, the temperature of the water would need to be ~200 F for a significant change in ion concentration. Thus, ion concentration would not be expected to be significantly greater at 140 F, in this case. Also, note that as solids dissolve, they do not settle. Once dissolved, the metals are no longer suspended solids, but instead are ions in a solution. For dissolved metals and ions in solution, a settling tank will have no effect on their concentration.
Settling of sediment in water heater:
Solids that do not dissolve will settle in a tank with low turbulence. Most likely, there are less suspended solids in a the hot water line after running through the water heater than the cold water line. The water heater definitely serves as a settling tank, as for many hours of the day, the tank sits idly. This allows the heavy, solid sediment to settle. Note that sediment is not the same as ions. Ions are dissolved and are uniform in solution, since their molecular mass is very near to that of water molecules. Sediment has a molecular mass much greater than that of water, making it fall to the bottom of the idle holding tank. Thus, a settling tank will provide a more pure water stream, but only with respect to total purity. The sediment is not measured by a TDS (total DISSOLVED solid) measuring device.