The small inexpensive instant hot water heaters for under the sink are intended to be used with your COLD water supply, not the hot, because it is intended to provide you with the equivalent of an instant kettle, i.e. for tea or for cooking. You should never use your hot water supply for anything that will be ingested, the hot water heater tank and pipes leach dissolved metals and chemicals into the water. Not a problem for washing etc., it's just not a good idea for eating.
That said, there is no reason why it CAN'T be used to "boost" your hot water supply instantly, but you may be disappointed with the flow rate on the ones that go under the sink. They come out like a drinking fountain, again because the INTENT is for making a cup of tea or not waiting to boil water for noodles. If you DID do this for your hot water supply, it would still have a thermostatic control to limit the maximum temperature, so one the hot water gets to it, it just works less or turns the heater elements off.
The other way to deal with this issue is to have another pipe run back from your furthest faucet to where the water heater is, and you install a circulation pump that runs hot water through your pipes throughout the house. You put that pump on a timer so it starts circulating the water just before you wake up and stops once everyone leaves the house, then starts again when people get home. If you do this, take the time to insulate the pipes as well. In fact if you have not already insulated your pipes, I would start there, you may be pleased with the results of just that step alone.