I've only done a couple of time trials, the recent Mercedes one was my first actually, so I thought I'd give this one a shot as well. After about an hour fiddling with a setup and ending up far from where I started, I found something that got me down to 1:20.8, and had intervals that indicated I could get to 1:20 flat but keeping the back end in check is the definitely a challenge. If anyone is interested I can post it later when I get home. I use a DFGT by the way.
I would suggest downloading a replay of someone who is 1/2 to 1 second faster than you to give you something to chase and show you where you are losing ground. Often it's all in one corner and the following straight.