I raced Austria in 2012 which was the hottest on record (and the first non-wetsuit swim). I raced in a full aero lid and wouldn't hesitate to do the same again. There are some hills, but they are short and sweet with a long payback. The surface is awesome. Basically, it is a wicked fast course and you'll be going at speed pretty much the whole time and making your own wind. Austria (and every other IM course) benefits from the full aero treatment.

Very jealous, Lake Worthesee is something else. I'm working on the missus to let me go back and race again as my 40th birthday present.

One more thing - RING THE BELL!!!! You'll understand when you get there.

I did IM austria '09 with a Giro Advantage and it was ok on a hot day; now I own a Rudy Wingspan, don't know if it's any more aero than the Giro, but it feels much cooler and more comfy.
remember that "tailed" helmets are more aero PROVIDED THAT you aren't often tilting your head forth and back, otherwise you're better off with the Air Attack

PS to your question about which aero helmet, it is difficult to recommend anything because it varies so much person to person which aero lid is fastest. But two which usually test pretty well are - Specialized McLaren TT and Louis Garneau LG-P09. Other fair bets if you can't get to the tunnel or do track testing are Bell Javelin, Giro A2, potentially Giro Selector. Lots of threads on here about this. Hearken to whatever Brian Stover says (Desert Dude).

thanks everyone. specialized is a steal at only £400!!!!

Yeahhhh!! Which is why I picked up my Bell Javelin in a special offer from a German site (delivered to UK) for about 70 quid! ;-)

I think, with the Bell Javelin or any helmet with a bit of a tail, much depends on where you hold your head and how well the tail fares onto your shoulders. I can't afford to go to the tunnel or test on the track (although we have an option now in the UK with Xavier Disley at aerocoach), so I tried a few helmets and based my choice on how it fits. I don't know for sure if this is a good option for me, but it looks like it might be. If my head was sticking up, I might consider something else, but then again if it was I'd be better off sorting out my position and learning how to shrug!