That weight difference would be worthwhile I think ! .
According to some info I dug up, the weight of a piston for a R100 airhead is
570 gr (http://www.zeebulon.de/Mot/bmw_allg.htm#Kolbenbolzen
) , and the one for an 1150 GS is 585 gr....(http://www.advrider.com/forums/showthread.php?t=836329
Mind you I did not measure myself.
But that would be not too big a difference in stress if you are not constantly bouncing of the rev-limiter.....