These results would disagree with your assertion:
http://www.firingsquad.com/hardware/...nce/page12.asp
http://www.xbitlabs.com/articles/vid...crossfire.html
http://www.hothardware.com/viewartic...&articleid=791
SLI gets slaughtered at antialiasing modes above 4x and no it doesn't have "better" AA.
Adaptive Antialiasing does exactly what nVidia's Transparent Antialiasing does and that is work on an object or polygon that passes alpha test and then has edge antialiasing done on it. However, ATi's adaptive antialiasing (quality mode) takes a much smaller hit than Nvidia's transparent super sampling.
Nobody said it matches it but the difference between the two isn't very pronounced nor is 8xS very practical for 90% of the new games out there at a decent resolution. Even 8xS isn't full supersampling, it's 4X multi-sample with a 2X super-sample (4xRGMS + 2xOGSS).
Actually 6xAAA takes a very minimal hit on ATi cards:
http://www.beyond3d.com/reviews/ati/r580/index.php?p=13
At 1600x1200 4xAAA/
8x AF X1900 gets: 93 fps
At 1600x1200 6xAAA/
16x AF (increased AF!) X1900 gets: 88.3 fps
Only in 8xS mode and unfortunately that mode takes a substantial performance penalty.
HQ AF, much better AA performance in Crossfire vs SLi above 4xAA/A, HDR+AA.