Let's do the math and find out, shall we?

The stoke on my 360 is 50.7mm and for argument's sake, we'll say I want to spin that baby up to 10,000 RPM. To get the speed of the piston, we need to do some math to first get the piston speed in meters per second.

First, divide 10,000 RPM by 60 in order to get RPS and we arrive at 166.66 RPS. So now we know that for every second that elapses, our crank shaft completes 166 and 2/3 rotation. Obviously, for every rotation of the crank shaft, the piston moves down and then back up again, covering 50.7mm x 2.

So now we multiply 50.7 x 2 x 166.66 and find that every second the piston travels 16,900 mm, or 16.9 meters.

Using the standard formula for calculating kinetic energy (KE = 1/2M*V² where M = mass in Kg and V = velocity in m/s) we find that a 210g piston traveling at these speeds has 29.98 joules of energy whereas a 221g piston has 31.56 joules.

Conversion from joules to ft/lbs gives us 22.11 ft/lb for the 210g piston and 23.28 ft/lb for the 221g piston. So you're right, not a great deal of savings.

That said, the main purpose of the piston purchase wasn't to reduce weight. I just happened to find it to be a pleasant added benefit.