I'm not a pro, but have read some on tankless systems. Most if not all have some sort of flow restrictor built into them to slow the water down so that the thing can raise the water temp enough. The tempering valve makes sure that rise isn't too high by mixing in cold water to the desired temperature, if it gets above the setting. In the winter time, my water is much colder, and my tepering valve never adds cold, thus I am stuck with the flow designed in the restrictor. In the summer, I get a better flow because it is mixing unrestricted cold to the hot to get the desired set temp.
I don't think that a new tempering valve will change the flow any, it may react quicker to spikes that can happen with changes in flow on a tankless system.
Don't know about the location of the valve or if it would account for anything. It is possible that they say to have it lower than the outlet of the tankless system to help prevent convection flow. Since the heater only turns on with a flow caused by having a valve open, it probably doesn't make any difference in your situation. Again, I'm not a pro, just my thoughts.