All you say of the (current) Generation whistles should be set to change. British pipe-maker Brian Howard, who was out here for the (Australian) National Folk Festival, in Canberra over last Easter, told me that he is redesigning the Generation head. They have no plans ... or even manufacturing patterns ... for their past heads (which have deteriorated as the mould suffers wear and tear) - so he is starting from first principles. I hope this means we will soon see Generation whistles with accurate, responsive head joints and consistent quality control!
All that said ... you should have tried to play anything listenable on the larger Generations when I first encountered them (c. 1964). Anything larger than the 'D' was so unresponsive as to be unplayable. (And, that was pretty early into their plastic heads ... there were still shops with old stocks of the cylindrical brass Generation whistles with lead fipples! I knew people who sought them out - they sounded great ... not too good for your health! One was left to me by an old player, Herb Gimbert, who died at 88. I occasionally play it - on its own, since it is in old ["high" or "Kneller Hall"] pitch, where A = 454 Hz - so I have covered the mouthpiece with 'shrinkfit' plastic to minimise contact with the lead mouthpiece.)