In my experience the difference between curve neck and straight neck ZE manifolds were minimal.
Also, I'd like to point out and question anyone who says they felt differences:
Did you change chips/flash your ecu to get the correct VRIS points? The KL31 VRIS and Curve neck VRIS are not wildly different, but they are a fair ways off.
4000 - 4800 - 6800
vs
3750 - 4250 - 6250
Whatever you end up doing - make sure VRIS points are set up otherwise you'll definitely feel that in your powerband
Don't know how much you could consider this proof but:
140 - 200 (km/h) KL47 manifold 21 sec
140 - 200 (km/h) KLG4 manifold 20 sec
Only the manifold changed between runs, i'm running an mx-6 gearbox.
When doing the KLG4 run, it was warmer, i installed leather seats which where heavier, there was more gas in the tank (about 10 liter) + I had two gearboxes in the trunk. So i would say the KLG4 performs better.