I suppose it's possible. I think the stock Riva has a little more mah than the stock eGo so I suppose it is possible that it hits just a little harder off of a fresh battery. Even if that is the case, the performance would almost certainly equalize once the battery drains a little. What is also possible is is you have had the eGo batts longer, their performance may have started to degrade a little so they don't compare as well with new batts.
I agree about the button. The Riva button is better.