There's a lot of good points in this thread for you OP. I know your situation sucks, but just collect yourself and try to negotiate with XFX. I know they're not offering you what you feel you're entitled to, but they are making attempts to work with you, and truth be told, you'll probably never see that XT. This is why I love EVGA. Twice I've had cards fail that were no longer in production, and EVGA supplied me with a newer card of slightly higher performance (latest issue was awhile back with a 7800GT that died and they returned me a 7950.)
If I were you, I'd try to calmly suggest some alternatives to their solutions. For example:
1) Show them the price difference between OC'd 4890's and 5850's for other vendors. If it's close to the $100 mentioned, offer them the $100 for the 5850 upgrade. You could also barter for a 5770 replacement. Sure it's slightly slower than a 4890, but a little more than the $130 they suggested for a 5850 upgrade would put you in good position for a nice 5770 CF setup. (Might be a little pointless for your WC though)
2) Propose the idea of maybe taking the 4890 non-OC'd as a replacement and possibly some kind of slight reimbursement. Maybe a low dollar amount non-expiring credit for a future purchase of one of their cards.
It's really out of XFX's hands when it comes to supplying you with a replacement for a non-production card. I'm sure they would work with you. You just have to show willingness to accept a reasonable offer, and go about it in a calm and collected manner. Obviously these types of situations make people angry, I know I would be too, but respect and courtesy go a long way in getting you what you want when dealing with a manufacturer. It's a two-way street. Show them that you understand their position and they're more likely to show you that they understand yours and work toward a happy ending for both parties.
As someone mentioned here earlier, their pricing for replacements is internal and independent of the retail channels. It's why even when you go through EVGA with the step-up program, their prices are higher than what you will find at online vendors. Just try to look at it this way: If you sold the 4890 XT and upgraded to a 5850 how much would you pay? Chances are that the total in the end wouldn't be that much different than taking their $130 offer. But try to offer them the $100 or even say $115 or $120. Worst that can happen is that they say no. If it comes down to taking the stock 4890 and getting nothing else, calmly explain to them that while you appreciate their intent to rectify the situation, you feel that you were slightly shorted in the process. Tell them that you hope to have better experiences with them in the future, even if you're completely soured to the idea of buying from them again. If they think that you're a loyal customer that feels their service was a bit short of what you expected, you'd be surprised what they'll do to keep you as a buyer.
-edit-
Somehow I missed Chai's post and the OP's response, kind of a repetition, my bad.