How to overclock in multi GPU?

[linda7788]

My hardware is:
CPU: Xeon E3-1230 V2
MEM:32G
Motherboard:Asus P8Z77 WS
GPU:HD7970*4(Not CrossFire)
System:Win7 x64
HashcatLite Ver:0.10
CCC Version:12.8

1.I set Performance->AMD OverDrive->High Performance GPU clock settings to 1050Mhz.
   
2.Run hashcatLite "oclHashcat-lite64.exe --custom-charset1 ?l?u?d --pw-min 8 --pw-max 8 --gpu-loops 512 b8631797bbe503304112f6987c2a8581 ?1?1?1?1?1?1?1?1"
3.press s to show run status.

HashCatLite show the GPU info is:
Device #1: Tahiti, 2048MB, 1050Mhz, 32MCU
Device #2: Tahiti, 2048MB, 1050Mhz, 32MCU
Device #3: Tahiti, 2048MB, 1050Mhz, 32MCU
Device #4: Tahiti, 2048MB, 1050Mhz, 32MCU

The crack speed is:
Speed.GPU.#1.: 9050.1M/s
Speed.GPU.#2.: 7972.7M/s
Speed.GPU.#3.: 7979.5M/s
Speed.GPU.#4.: 8004.6M/s
Speed.GPU.#*.: 32910.2M/s

Only GPU 1# overclock and improved speed.
Why GPU 2#/3#/4# are slow then GPU#1?How to improve them?