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I have a 1999, Kawasaki KDX 220. I’ve been needing to put a new piston in it. And I need to know if the high compression piston has any benefits over the stock medium piston. Or if the stock medium piston is the way to go.

Where are you seeing a piston that is high compression? I am just used to the normal piston.

Yeah, I've never seen a high compression version of any piston offered for the KDX in all the years i've had one.

The Wossner and Wiseco pistons are forged aluminum, which is more durable but has longer warm up requirements before getting into the throttle. On a 220, you absolutely never want to use the OEM '97-'05 cast pistons, they are not made by the same company that made the much more durable 1986 - 2006 KDX 200 piston. The factory 220 pistons will blow up on you without warning.
The only thing to alter compression on a KDX within reason is milling the cylinder head and not re-chambering it back to stock volume. People mill the heads to get a tighter squish band clearance, but then usually if done properly will rechamber the head and take some material out of the dome to get the volume back to normal. You don't really want over 200 PSI compression on a bike that runs pump gas and is kickstart especially.

I very much prefer the Wossner postons over Wiseco, because they have coated skirts to make a smoother running piston that wears less on the cylinder wall plating, as well as the fact that they come in A (69.00 bore), B (69.01mm), and C (69.02mm) sizes to compensate for bore wear on the plating. Wiseco only offers 69.00mm bore and 70.00mm 223.2cc "big bore" @ 1mm oversized from factory.
Wiseco makes good pistons, and are even based out of my home state of Ohio here in the USA, but I prefer the Wossner for the two reasons stated.