1.5 vs 1.6 rockers

Adding 1.6 you are also putting more drag on your timing chain and all components in between.
1.6 Will change the duration because you will be taking up the valve lash faster. but not much. the more lash the more change.
1.6, You should check your piston clearance and also the max spring compression.
Adding 1.6 will give you more power if you have smaller valves. or shorter duration or smaller lift than optimal.