Techie stuff like polypro linings and titanium neoprene can reduce the thickness you need for the same warmth.
Also, if you surf in windy conditions, single lined neoprene on the outside will be warmer, because it sheds water so less wind chill, although the suit won't be as durable.
Bright coloured neoprene is usually colder, because it's white rubber not black, which is a shame, because I like lime green wetsuits.
There's some small ozzie outfit making suits out of a different material that can give a 5/4 warmth with a 2mm suit. I'll search out the name later - they are supposed to be really good too, as used by Ozzie special forces etc.
Ozzies are better at making wetsuits than they are at rugby.
