That all sounds right to me. I have raced a lot with 1515, 1512, 1509 motors, all on 4s and all in the 1700-1900kv range. 1515 2.5d in this video...
http://www.youtube.com/watch?v=I7c-i...e=channel_page
I have also been racing a 1509 (1820kv) recently with some success. It's been running hotter than I like the last few races, with temps as high as 180 (and thus not as efficient as I like) but it's worked really well at some other races, not getting over about 130. 1/8 buggy is pushing what this motor can do, especially if the buggy is NOT light. (My Caster was only 7.5lbs race weight with the 1509 setup) It has plenty of power, but it's not proving to be as efficient. I think a lower kv/6s setup would work pretty good though.
Your 1515 can run great on 4s... just needs more gear. You didn't say what you geared it, but with the kyosho diffs and 46t spur, you need to be at least 16 on the pinion to start getting "fun" speed out of it. Might want to go back to CEN diffs and try it again. :)
I'm going to start running my 1515 again too. :)
Feigao = junk lol