I recently visited the nursery facilities at the Royal Botantical Garnes, Kew, London. They have a large programme, propagating and growing many Neps (both highland and lowland). Most of the highlanders were growing in a computer controlled glasshouse with refridgeration, so that night temperatures were at 50 F (10C). However, due to lack of space several highlanders were growing in the sister glasshouse for lowlanders (night temperature 65F (20C)). Suprisingly there was no noticable difference in growth with the N. rajahs under the two conditions.
This was an expensive facility, beyond the means of the amateur grower, so all other conditions (light, humidity etc) were near perfect. So whilst N. rajah probably won't do well at 75F, I don't think it isn't necessary to chill it down to 45-50F to get good growth, 60-65F will probably do it. N.rajah is an amazing species, and having seen the temps that Kew get it to grow well at, I will probably get one soon. Even in chilly England I would struggle to get summer night temperatures much below 65F without some AC system.
Just my $0.02
Vic