@Apothem: Nope no crossfire, did try it though without any observable performance gains or losses. I also tried to plug in the thinner Sapphire 7950 into PCIe slot #1 and the larger Asus 7950 into slot #2 (instead of slot #3) but no gains here either:
Good GPU: (Sapphire)
Evil GPU: (Asus)
An interesting observation is that if I plug my two Asus 7950 into the PCIe slots #1 and #3 the first Asus 7950 has a voltage of V: 1.090V while my Sapphire here uses V: 1.169V. The conclusion here is that my problem does not seem to be caused by an upper voltage limit, since V: 1.169V + V: 0.850V (Sapphire+Asus) is larger than V: 1.090V + V: 0.850V (Asus+Asus). Observing this eliminates (almost) all my suspicions that my motherboard might be the root of all evil (was very unlikely anyway, since its brand new and was not the cheapest available).
And since all cards deliver individually excellent hashing rates (595Kh/s for both Asus and 630Kh/s for Sapphire), I started to think that software might be the reason why I can't increase my performance with the number of graphic cards linearly.
Another option might be that I need those powered risers (PCIe extension cables) to solve my voltage issues; since I don't have those cables available right now I'll continue to pray to the ASIC god to increase my hashing power through a miracle .. damn. 
Good GPU: (Sapphire)
Code:
GPU 0: 294.3 / 274.6 Kh/s | A:5 R:1 HW:0 U:2.14/m I:20
76.0 C F: 85% (4016 RPM) E: 900 MHz M: 1250 Mhz [b]V: 1.169V[/b] A: 99% P: 20%
Last initialised: [2013-05-01 22:14:20]
Intensity: 20
Thread 0: 276.9 Kh/s Enabled ALIVE
76.0 C F: 85% (4016 RPM) E: 900 MHz M: 1250 Mhz [b]V: 1.169V[/b] A: 99% P: 20%
Last initialised: [2013-05-01 22:14:20]
Intensity: 20
Thread 0: 276.9 Kh/s Enabled ALIVE
Evil GPU: (Asus)
Code:
GPU 1: 253.5 / 274.6 Kh/s | A:2 R:1 HW:0 U:1.43/m I:20
37.0 C F: 8% (1230 RPM) E: 300 MHz M: 150 Mhz [b]V: 0.850V[/b] A: 0% P: 20%
Last initialised: [2013-05-01 22:14:20]
Intensity: 20
Thread 1: 230.1 Kh/s Enabled ALIVE
37.0 C F: 8% (1230 RPM) E: 300 MHz M: 150 Mhz [b]V: 0.850V[/b] A: 0% P: 20%
Last initialised: [2013-05-01 22:14:20]
Intensity: 20
Thread 1: 230.1 Kh/s Enabled ALIVE
An interesting observation is that if I plug my two Asus 7950 into the PCIe slots #1 and #3 the first Asus 7950 has a voltage of V: 1.090V while my Sapphire here uses V: 1.169V. The conclusion here is that my problem does not seem to be caused by an upper voltage limit, since V: 1.169V + V: 0.850V (Sapphire+Asus) is larger than V: 1.090V + V: 0.850V (Asus+Asus). Observing this eliminates (almost) all my suspicions that my motherboard might be the root of all evil (was very unlikely anyway, since its brand new and was not the cheapest available).
And since all cards deliver individually excellent hashing rates (595Kh/s for both Asus and 630Kh/s for Sapphire), I started to think that software might be the reason why I can't increase my performance with the number of graphic cards linearly.
Another option might be that I need those powered risers (PCIe extension cables) to solve my voltage issues; since I don't have those cables available right now I'll continue to pray to the ASIC god to increase my hashing power through a miracle .. damn.