A friend of mine sells lots of crossbows. I assemble them and test. I have a z355 and love it. He shoots a 380 I think, and it is a smooth runner too. The 400+Fps bows can be hit and miss with limbs cracking. Have not seen any limb failures on killer instinct, but have seen some on barnett.
Seems like everyone who buys a bow from him is worried about speed because that's what is advertised. Killer instinct bows are pretty nice compared to other brands. They are affordable, decent quality, and they just plain shoot. Mine will robinhood arrows almost every time if I shoot at the same target. My friend's will do the same. I have a dime sized hole in the center bull, because that's where the arrow always goes. It's good enough that I take head shots if they are available.
I think you will be fine with a 415, but I would use a heavier head or arrow to make better use of the power. It will be quieter and smoother.