I've had my Billion 8900AX-2400 for a while, it's a great piece of kit! I've recently changed ISPs (from Plusnet to Andrews and Arnold), mainly because I was getting annoyed with them for various reasons.
The router is sync'ed at 79287kbps down (20000kbps up), but I have noticed some unusual behaviour when IPv6 is enabled - these results are from their speedtest (http://speedtest2.aa.net.uk/plain.html).
IPv6 disabled
Download: 73.06 Mbit/s
Upload: 20.14 Mbit/s
Latency: 18.64ms, 1.98ms jitter
IPv6 enabled
Download: 66.88 Mbit/s
Upload: 19.29 Mbit/s
Latency: 18.58ms, 1.86ms jitter
These results are consistent, a loss of around 5Mbit/s when IPv6 is enabled. This isn't a lot, but when your paid for decent kit and connection you expect it to perform

Any one have any ideas?
Craig