It depends highly on what turbochargers you use, and how much boost you run.
Definatly go with a forged crank engine though. Otherwise that will be your limiting factor later on. Also try to use the high flow heads from a turbo engine. If you use a used crankshaft, have it reground (if needed), or polished. Use quality bearings (matched to the regrind size). The main and rod bearings don't have much surface area on that engine. You need to use good ones, and keep them well oiled (with a slightly heavy oil).
Does someone accually make oversized pistons in that size? Or are you talking about getting ones custom made? Oversized pistons will definatly help with spool time, and torque.
With that piggyback, make sure that you are always pulling back fuel, and not adding it. Go with slightly larger injectors then you need, then just pull back fuel on the piggy back. Try to get the injectors closely matched to your power output. Try to tune too far with that piggy back, and you will start getting check engine lights. But a piggy back system is a much better place to start then standalone. Standalones are a bit harder to tune.