Cards made by the designer are usually not as good as the cards made by partner manufacturers, for two reasons:
MSI, Gigabyte, Asus, Chaintech etc build the product using chips/components they buy in bulk, they save money from buying 1 million memory chips clocked at 600MHz and then installing them in cards that require 600MHz and 550MHz, they don't buy memory chips for 500MHz, 550MHz and then 600MHz...
If you look you will see that cards like the Asus ATI Radeon 9600XT is supposed to have a memory speed of 450MHz but because Asus only had memory chips of 500MHz they increase the speed and call it the Asus ATI Radeon 9600XT Platinum or something.
The other reason is, partner manufacturers usually have a lot better cooling and thus allow for better overclockability - they also come with their own utilities and maybe even their own more stable/faster BIOS.
[size=0.5]Full Aluminium ThermalTake Case
AMD Athlon XP 3200+ *400 FSB* Barton (2.2GHz)
ThermalTake Volcano 11+ Xaser Edition Air Cooling
1024MB Corsair TwinX XMS 3200 (Dual Channel 2x512) 2-3-2-6
256Mb XFX 7800 GS EXTREME
2x WD Raptors (10krpm 8MB Cache) 36.7GB using RAID -0
Video Logic SonicXplosion 5.1 Soundcard[/size]