I have a range of gens – one in each key. Two play really well, the others don’t, though the bluetack tweak helped (all but the C – I still can’t get the head off that one). Last night I put them all out on the coffee table and noticed that they all have a sort of bump on the lip of the fipple (hope I have my terminology right). On the two that sound good, that bump starts noticeably futher back – that is, there’s a longer flat space between the blade and where the bump starts. I’m thinking of sanding, filing, or otherwise trying to create a longer flat space on the whistles that don’t sound as good.
If you can remove the bump cleanly and leave an accurate, smooth ramp that’s still the correct angle, I would recommend that you go ahead and do it. On the other hand, if the result will be approximate, not flat, rough, etc., I would say leave it alone.
My experience has been that a perfectly flat, very smooth (polished, even) ramp often does help clean up the sound and focus the voice. (And yes, I have done this with Generations.)
It gives you first hand experience so you’ll know what (not) to do next time, and it’s fun. I’ve sandpapered a few heads and the first one was a disaster, but I learned from that how to do the other ones.