I have an opteron with older slower memory that I reproduced the pipe tests on to see if it was any different on a 64bit system. I'm not going to paste the full results but here's a couple of data points:

linux-2.6.24
64[writer]: 97.235 wall (2.031 usr, 68.674 sys), 10.531 Mb/sec
1024[writer]: 13.300 wall (0.145 usr, 9.039 sys), 76.991 Mb/sec
65536[writer]: 3.068 wall (0.001 usr, 1.718 sys), 333.766 Mb/sec

FreeBSD 8.0-CURRENT undermydesk (no cpu switch patches though)
64[writer]: 53.163 wall (1.057 usr, 42.083 sys), 19.261 Mb/sec
1024[writer]: 5.325 wall (0.118 usr, 4.146 sys), 192.284 Mb/sec
65536[writer]: 0.567 wall (0.000 usr, 0.130 sys), 1805.509 Mb/sec

So on this machine we start of 2x as fast and end up 5.5x as fast. The numbers pretty much follow a curve through those points. This verifies the data taken from the old 32bit HTT machine they tested on. I don't intend to post configs and so on as the original lkml thread is plenty rigorous enough.

I forgot to mention earlier. The FreeBSD Alan Cox has committed super-pages! We're seeing some great gains from that. This allows the kernel to automatically use large TLBs for conforming regions of memory. It has a component that ensures that large, contiguous, chunks of physical memory will be available to support this. There is also a defragmenting/compacting piece. There's some great work going into FreeBSD 8.0 already!