Julian,
I side with the flat top guys and I would steer away from hypereutectic. Rather, I would suggest a good set of forged pistons; the few dollars extra will be worth the peace of mind.
The thing that properly dimensioned flat tops will give you is quench surface. This is a subject that is not well understood (by the masses anyway) and seems to be lumped in with things such as voodoo and other arcane sciences. As near as I can explain it, the flatter, smoother and plainer the surfaces surrounding the compressed air/fuel charge, the less likely that detonation will occur. The reasoning ranges from removing hot spots to controlling local pressure variations to removal of hiding spots for demons & ogres. This means that you can run higher compression ratios, obtaining more efficiency from the explosion, without encountering the destructive phenomenon of detonation (the explosion occurring before it is supposed to.) In the old days, one would just use a higher octane fuel (does not detonate as readily as a lower octane fuel: lets put diesel on the low end and alcohol on the high end for descriptive purposes). Of course that is not so easily done anymore. The industry answer was to lower compression. So, these have been the main two variables that have been manipulated. However the topology of the combustion chamber is a third variable that can be manipulated with satisfying results.
For completeness: In naturally aspirated engines, these three are pretty much it; however, with forced induction, inter-cooling (and its many semantic variants) along with water or water/alcohol injection also come into play.
Regards,
Lynn