These graphs compare the initial values from the 2011-12-15 kernel (blue) to the current IPv4 (red) and IPv6 (green) values of 2012-04-08 showing improvements.
On IPv6 UDP checksums are mandatory, so we did not turn them off for comparisons.
In general we do see improvements here for IPv6. In most but not all cases also for IPv4. IPv6 is still behind the performance of IPv4. The general comment also mentioned on TCP loopback applies here as well: The range of data was once again high, as was the interquartile range (IQR), but has been stabilized. This is a general loopback problem that should be addressed independently of IPv6.