well i used to have a 3G, and changed the battery myself about 3 times, but never as there am 1100ma version (dang it!).
You wont get 15 hours out of it, i can tell you that,the current ma in a 3G is about 850ma, so its not really gonna have a big impact.
A complete charge cycle would be around 2 1/2 hrs, thats will get the battery to 80% it take about another 40mins more to get the battery to a full 100%.
Whos making the battery, is it one Apple Corp are selling, if it is i wouldnt bother. just buy a replacment to your current one.