2x7990 or 4x7970

[random2]

Some time passed and I got a PC!
But the speed test shows less than expected:

Hashes: 1 total, 1 unique salts, 1 unique digests
Bitmaps: 8 bits, 256 entries, 0x000000ff mask, 1024 bytes
Workload: 256 loops, 80 accel
Watchdog: Temperature abort trigger set to 90c
Watchdog: Temperature retain trigger set to 80c
Device #1: Tahiti, 2048MB, 1100Mhz, 32MCU
Device #2: Tahiti, 2048MB, 1100Mhz, 32MCU
Device #3: Tahiti, 2048MB, 1100Mhz, 32MCU
Device #4: Tahiti, 2048MB, 1100Mhz, 32MCU
Device #1: Kernel ./kernels/4098/m0000_a1.Tahiti_1084.4_1084.4 (VM).kernel (322816 bytes)
Device #2: Kernel ./kernels/4098/m0000_a1.Tahiti_1084.4_1084.4 (VM).kernel (322816 bytes)
Device #3: Kernel ./kernels/4098/m0000_a1.Tahiti_1084.4_1084.4 (VM).kernel (322816 bytes)
Device #4: Kernel ./kernels/4098/m0000_a1.Tahiti_1084.4_1084.4 (VM).kernel (322816 bytes)
...
Speed.GPU.#1...: 5313.1M/s
Speed.GPU.#2...: 5329.8M/s
Speed.GPU.#3...: 5318.6M/s
Speed.GPU.#4...: 5312.8M/s
Speed.GPU.#*...: 21274.4M/s

I saw HD7970 test on another post - the speed was about 8000 M/s. Why I got 5300 on the same HD7970? Maybe I need something to adjust? Thank you.