i'll throw my input out there.. I have jhautz 1515, and he's got my old 1512. I simply couldn't gear my truck to keep the 1512 temps down. I believe I was around 16/46 on my LSP, and without a fan on the 1512, the motor got too hot to touch in under 5 minutes, on a 90 degree day. I've got the 1515 now, and also using the same gearing.. the motor was barely warm without a fan in the same conditions after 10 minutes, and with a fan on the motor, I couldn't feel any heat at all. I lost about 10 mph, but can easily gear the 1515 back to the ~40-45mph I was at with the 1512, but won't have any heat problems at all. My Quark runs alot cooler as well. before you couldn't hold it, now I barely feel any heat.
depending on how you can gear your truck, I think the 1512 2.5d is too hot on 5s, but would probably run excelent on 4s. a slightly lower kv 1512 would probably run great on 5s as well.