[Mike Halliday]

Hot water will cool in seconds when it touches a cold surface so unless it creates more activity/ strength within the solution I can't see how using hot makes any difference

The Oxygen bleaches used in cleaning are excelerated by adding strong alkalis, although I don't know how hypo will react to this (so don't recommend it)

[Roger Oakley]

I don't think anyone mixes it hot to be honest mate.

Normally use tap water

i have often applied it to surfaces through my hot pressure washer after cleaning at a 5% mix through a venturer which gets rid of any slight stripeing obviously this is not something others have been doing

As you say hypo will help get rid of stripe and lance marks, but no advantage with it being mix in hot water,
Mixing it in hot water in the winter, again no advantage as the hot water cools/dry's quicker then cold water when hitting the surface so evaporation is quicker, and you want the chemical working on the surface not drying out.

Adding oxygenated bleaches to hypo is ok, but it is just a short boost for the hypo, doesn't seem to clean any better or any worse IMO. Just be careful of what you are mixing & applying in.