Compression and displacement are different things and the math - well I was never good at that myself.
I have the alloy 535 and the forged 8.5:1 "acralite" piston.
The stock head is lapped to the cylinder.
There are several benefits: better heat dissipation, more torque and power, I would be surprised if I would see the need to decoke as much as with the standard setup.
Downside is that I need premium gas and had to fettle with the clutch.
As it is a forged piston, I just broke it in quickly and switched to synthetic oil after about 700 miles. I was also breaking in a new head otherwise I would have switched after 300-400 miles. I do an early (50-100 miles) oil change though, to get the inevitable shavings out as much as possible.
As far as I understand, cast pistons are a different matter for various reasons.
But I don't have any interest in these so I never bothered to find out the gory details.
Peter