I just had a quick look with some digital calipers and a bit of macro photography - and the plastic caps I think press the seals better. The plastic cap on the body is approximately 0.1mm shorter overall than with the metal cap.
Like I said though - if you're finding the shocks leaking there's got to be a cause and the most obvious other than damage (which I'm sure you've checked for) would be the seals not getting enough pressure - adding a 0.5mm plastic washer or similar should help matters. I don't think the plastic caps are a problem - but like I say, I've not used them myself yet.