I agree with you, but the question is what happens when you overclock the 5870?? It goes ahead again, maybe not 15% but when your at the margin of 25 vs 30fps thats the difference between playable and not playable and with no crossfire support for eyefinity in sight yet I felt personally it was a safer bet to get the 5870.
I had crossfire 4850's and upgraded to the 5870 and I was very happy to get away from dual cards and could not justify the move to a 5850 as it probably would have been not as powerful as my current 4850 crossfire.
To tell the truth there have been a few games so far that sit at like 30fps when on high settings, so I think the 4850 would not have made the cut. The 2GB Six maybe would have been worth the wait but who knows how expensive that card will be, and it may require some $$$ for adapters too.
I happen to have 3x U2410 all of them support display port but we do not know if mini DP --> Full DP will be included in the box yet.
I think I will hold out for the 5870X2 and make that the next logical upgrade from a single 5870, I cant even fit another one of them in my case in crossfire (P182) and a P45 mobo like I use drops from 16x to 8x on each slot in crossfire and in high resolutions that has been shown to decrease performance.
I havn't noticed people getting higher overclocks on the 5870 except for the people who have performed volt mods and replaced coolers and this could be done on the 5850 as well.
My Asus 5850 does
850 Mhz core and 1251 MHz Memory at 1.1v
976 Mhz core and 1251Mhz Memory at 1.187v
Thats pretty much as good as the 5870's I have seen on forums and there is more in the card I just chose to stop adding voltage.
The 5% difference from the extra shaders would be more like the difference between 28.5 FPS and 30 FPS. Your example of 5 FPS difference on a 30FPS game would be 16.6 % additional performance which as stated before is not the case on equally clocked cards.
I also upgraded from a 4850 and went from 1.1 TFLOPS of performance and 11 minute 30 second Collatz Conjecture runs to 2.8 TFLOPs and 4 mins 20 seconds with the overclocked 5850. Thats a gain of 2.5x. Gaming performance is also about that much better.
The 5850 on paper seems limited but all the limits are superficial. The 4850 had very real limitations for example DDR3 vs the DDR5 on the 4870 killed the bandwidth. The 1 x PCI-e 6 pin connector limited the card to 150 watts of power draw and ruined any overclocking.
However yes for some games every extra helps and if you have the extra money why not :D
You should consider the 5950 (5850x2) as your next upgrade step for all the reasons discussed and it will be a shorter card maybe even only as long as a 5870. The only problem would be if the card had less power connections available limiting its clocks which is a possiblity.