Hypereutectic
basically high silicon levels in the cast aluminum(*corrected*) alloy. Increases strength, melting point. Get most of the advantages of a forged piston, with the cheaper price of a cast piston - material costs are more than a standard piston, but still cheaper than forged
Seriously, if you want to increase the compression ratio, you should just go ahead and get custom forged pistons. Hypereutectic are strong, but forged is stronger. And when you are dealing with a explosive situation like a piston... might as well get a better piston.
as for increasing torque. . .
A higher compression piston should increase torque, it will also increase the chance detonation under boost unless you add race gas, or more fuel, or as BlindFreddie suggesting, water/alcohol cooling to the intercooler.
But if you just want low end torque, why not just get a different turbo, and a custom chip? You can get a smaller turbo, so that it spools quicker, it would give you the boost quicker, which would give you more torque at a lower rpm, it's be easier to change the turbo than to replace the pistons. Of course you would loose top end performance, since it wouldn't boost as much at a higher rpm... just and idea.