You're much better than I am. I have practiced singing whistle notes before but I have mostly given up on them. I'm much more interested in having a continuous range. Who cares if you can sing E7 if you can't hit A4? I still hit one once in a while for fun though. Not too long ago I hit E7(three octaves above middle C). It was the lightest faintest sound, barely a squeak. I can more easily hit D6. the problem for me is it takes so much time to find the correct placement I cannot hit whistle notes on cue, so I would never use it in a performance. The highest I will perform is A5 when I sing The Darkness' "I Believe in a Thing Called Love" at karaoke. Have you tried inhaling your notes instead? This usually gets me pretty high (in range haha!). It is probably bad for the voice though, so be careful.