I currently have two DrayTek Vigor2820 modem routers at two sites both with Telstra Business ADSL2+ connections. They each have 1mb upstream link. I have set up a VPN between them (dial in IPSec VPN) which is working but is quite slow.
DrayTek modem/routers come with QoS enabled by default, and by default it may route all traffic through as class 4, only giving 25% of the available upstream bandwidth while leaving 75% of upstream bandwidth unused. Disable QoS, and 100% of the upstream bandwidth will be available.