When I've used a sub 80°C steep it has certainly produced a hoppy beer but I don't think it has produced a more hoppy beer.
My understanding is that above 80°C you are potentially adding bitterness and boiling off the more volatile hop oils. The rate at which the aroma compounds are extracted and the rate at which they boil off are all related to the the temperature of the wort. At a higher temperature you will get quicker extraction but also more volatilization. There has to be a sweet spot to get maximum extraction and minimum volatilization but where that is will probably vary depending on the particular hop oils that you want to capture in your brew. Potentially the best method would be to do a long low temp steep but as I brew in the evening and it is usually well past midnight by the time I'm chilling my wort, I am not going to hang around for a really long steep at 50°C. I think you also risk leaving the wort at perfect bacterial growth temperature if you go too low for two long.
One thing that has made me worry slightly less about hot steeping was a recent addition of the basic brewing podcast. They interviewed a homebrewer who had attempted to make a decent non-alcoholic beer. He did this by fermenting the beer, then raising the temperature to 75-80°C which is the boiling point of ethanol for 45 minutes. Despite the smell of alcohol leaving the beer completely, it turned out to still have quite a bit of alcohol in it. I think hop oils would behave in a similar manner so hopefully there will still be a decent bit left in the wort.
Talking sense Simon.
I do think we are looking for a sweet spot, temperature/time. Or a line on a graph maybe, time v temp.
I once did a 1 hour steep at 60C with loads of hops, cos I over cooled the wort, I didn't feel it was very effective.