What chainset for time trials?

It's really up to you. You need gears that you are comfortable with. While being competitive in a TT may, optimally,require you to push the tallest gear you can for the time of the race in my view it would be silly to put big gears on a bike that you can't push at a normal cadence (i.e. ~90 rpm). If the compact setup you have is good for you on club rides it will do fine for TTs. The only reason to consider a different setup would be because whatever you have now doesn't have any gears that you can't spin at 110 rpm or so ... and I doubt that's the case.