What mobo is he using? You are running a 790GX chipset- the only way I can possibly see a difference is if he is using a 790FX chipset, and even then, the 8x PCI-e shouldn't be saturated by a 5770 or 4870... have you tried completely sweeping your drivers away, and verifying that you are running xfire, and not just using one GPU.
I just check my score (my single 4870 with a bit of an OC and my i5 OC) and my GPU score is around 10000 as well, which means you might only be actually utilizing 1 of your GPUs. I don't know how to verify that really since I've never X-Fire, but I would suggest running GPU-Z or something while you are running 3dMark Vantage and see if you've got utilization on both cores. My friend had an issue when setting up his 4890's in X-Fire where he had to completely wipe his drivers and follow a specific process to install them to run his X-Fire properly. Something to try at least... I don't know the process though, but you should be able to find it if you google search setting up X-Fire.