As PTFE doesn't conduct heat it effectively acts as a barrier between the head and the inlet thus keeping your intake temperatures lower. 8)
As a guide on a 172 where the plenum itself sits on the head (french!) and only using a 1.5mm thick PTFE gasket we saw reductions of around 4-6degrees on average on start, stop, roundabouts, etc and upto 8 on the more open stretches. However we would expect bigger gains on better setups (i.e. the GTIR and especially Hondas) where the inlet is more remote. On a friends clio 16v you could nearly rest your hand on the inlet manifold after driving for say 15mins after fitting the gasket - prior to fitting you would have been in A&E!! lol :yuck: