Yours certainly does shoot beautifully, Michael.
. And, yes, it would be hard to imagine how it might be improved on further. Nick has most certainly proved the effectiveness of 'O' rings and they're so cost effective. I think they really come into their own even more when experimenting with different bore sizes, where a parachute seal may not be available, as Nick has also found.
It would be nice if you could get a spare piston (wonder if they're easily available still, like the service kits they used to do for them?), convert it / have Nick convert it and compare side by side?
In fact, thinking a little more about this and considering how superbly your LGV shoots, I'd probably be more tempted to leave it alone for the time being. "If it ain't broke, don't fix it" and all that. But to obtain a piston and have it on "standby" for future use / experimentation. Don't know if you can do the machining work yourself or if you'd need Nick to do it to get the "squish" just right (in which case Nick would need your cylinder)?
Any progress / further thoughts on the 280?