Replacement cast pistons are much, much heavier duty than the stock TRW ones. That's why I went that route in doing my build. The pistons are holding up just fine, too. They're Silvolites..nothing special, but the skirts are about 1.5 times as thick, and the actual tops are much thicker, with better machined valve reliefs, overall better casting.
If you're going with a custom piston setup, you could use the SPI shape. I've seen a fair share of messed up pistons, mostly from detonation and broken ring lands. I've also seen cracked tops from the squish area not being measured correctly or even checked, due to the head being milled. The piston kept coming into contact with the combustion chamber wall, and over time it caused a lot of damage.
Larger than the SPI valves? I can see the exhaust size being that large, but the SPI intake valve is already larger than the aftermarket large valves that can be had for the 1.6 head. I'm guessing custom valves as well, along with larger seats.
Do you have any flow bench data regarding the SPI and HO intakes? I'd love to see that.