Is there a major difference between 2015 z28 and 2017 1le. And if add power to both to make 650 hp which one will be closer to the zl1 1le

The 2017 1LE is based on the Alpha platform (ATS platform). If you're looking for a vehicle that straight line will be closest to the ZL1 with the same horsepower, the 2017 should be quicker since it is approx 200 pounds lighter.

If you're into cornering, one of the best features of the sixth gen 1LE platform (on the SS) is the E-LSD, which is very similar to the C7 E-LSD.

I would also be concerned about the dropped valve issue on the 2015 Z28. I know two folks who lost their LS7s, and it is a concern of mine (I have a 2009 LS7 and I need to get the heads addressed to ease my mind).

Hope this helps - they're both really nice cars. Post up what you decide.