So, last week on Monday, I started a support ticket via XFX's website. It took them a couple days to reply. They asked what power supply I had and instructed me to reinstall the drivers. I did, but that didn't change anything. I responded that Wednesday and I waited a day for a response, but I got tired of waiting so I tried calling instead.
To my surprise, XFX HQ was only a couple miles from my house, so I was able to just drop off my 4890 for RMA that Thursday. They told me they'd test it and get back to me on Friday. Well, they didn't get back to me that quickly, but they called me on Monday and I was able to go pick up a replacement right away.
I was particularly worried when I saw the replacement had the same exact model number as the defective 4890. Turns out that the replacement was a reference design 4890, but with a black PCB, and 6pin and 8pin PCI-e power connectors.
Pictures! (Sorry for the fuzziness)
Pic1 - The replacement "ZDFC" 4890.
Pic2 - Looks like reference design.
Pic3 - Down near the bottom, that looks like the digital voltage regulating thing, I think. It doesn't look like the analog VRMs found in the previous gfx card.
Pic4 - 6-pin + 8-pin PCI-e connectors.
My PSU doesn't have an 8-pin PCI-e connector... I called them today and they were able to provide me with a 6-pin to 8-pin PCI-e adapter.
Now that I have the replacement 4890 up and running, I ran a couple quick benchmarks.
(stock 4830 / OC'ed 4830 / replacement stock 4890)
FC2 DX9 0xAA: 24.13 / 30.09 / 36.80
FC2 DX9 8xAA: 15.66 / 18.69 / 24.44
SF4 8xAA: 47.9 / 59.22 / 76.33
While the OC'ed 4830 was roughly 20-30% faster than stock, the replacement 4890 is roughly 50-60% faster than the stock 4830. That's more like what I had originally expected.
I'm not sure I'll bother doing more benchmarks. For the moment, I'm quite happy, especially seeing how helpful and useful XFX's support is. Now, back to playing ME2...
EDIT: I noticed something. Apparently, this replacement I got is V1.0, while the older one that I RMA'ed was V5.4. Just worth noting in case anyone cares...