Neu motor question (1512 vs 1515)

[JakeE]

I'm in the process of converting a Hyper 7. Battery is a 5S2P A123 pack. I had originally planned on using a Neu 1512 2.5d based on the recommendation from BrianG's motor selector. At that time I was figuring on a total weight of 8 lbs. or less. Realistically though, I'm looking at more like 8-9 lbs. at which point the motor selector tells me to use a 1515 1Y. Is this accurate, do I really need to jump to the 1515 series if my buggy goes over 8 lbs.?