Depends on the game.
90% of games will run on the A7 cores without ever needing the A15 power.
3D needs minimal CPU power, for example clamping the CPU on our current devices to single-core and 600MHz will have absolutely no effect on benchmarks like GLBenchmark or the Unreal Citadel.
However if the game does physics, A.I. computations, sound effects and who know what else, then the CPU will be loaded.
It also depends how threaded the game is. I don't think that games run more than 1 "heavy" thread, so in optimal conditions you'd only have a single A15 up while having the rest of the pairs staying on the A7 cores.
There is no definitive answer to your question. Just fire up CPU-Spy, reset the frequency states and go play a game and see the result. If you're on an A9 core phone then theoretically if you exceed the 1000MHz state, then that would be over the capacity of what the A7's could deliver and it would switch over to the A15.
Edit: pic removed humor is not allowed in xda
[Center]PHONE SLOW CLICK ME?
1 days 2 s4
peru
Sheffield
u could've ignored but I'll remove
even while gaming!
So Cal, California
Amman

Dublin/Paris(back and forth)
China
Linear Mode