Has anybody used one? I know a thicker gasket can be used but that alters port timing too.

I fitted an S3 one on my sons 2013 300 2 stroke, as it was such a pig to kick over. It had loads more compression than my 2014 300 2 stroke and he had just moved up from a 125 at the time.  It softened the power down a little. But as we had a full Bossi Arrow exhaust to fit we weren't that bothered.  

Edited December 20, 201510 yr by p@ul250

I put one of the gold S3 chambers on my 2013 as I was having issues holding on (realized later it wasn't bike, was Lyme disease) anyway the lower compression head makes the bike easier to start. Not easy but easier. Still grunts up slow hills like a 300 and will still jump when the throttle is whacked but the hit is less violent. I didn't find it necessary to change jetting. It may help the Beta's appetite for race gas, I don't know because I use VP C-12 anyway. I think it's a good mod for anybody below expert. I also recommend carbon reeds as they allow the bike to pull smoother and cleaner off the bottom.

I think I prefer the older sizes eg 270....they are a bit snappier

it's interesting how the manufacturers go up and down engine sizes over the years....I guess without change the bikes aren't as attractive?