I did not realise that you still had a stock wheels on the bike but your observation on the downhill speed is the same as I found. Speed went up on the downhill by a lot- Corners at speed became curves and average sped over a 30 mile ride went up by 2mph. And on top of that- The lightness of them gives an easier time uphill.

I have always said thet I do not like stock wheels and you have just confirmed the reasons why. Just enjoy them now.