There are many, many variables that affect observed "bandwidth" (or, more accurately, throughput).
The 2811 by itself is more than capable of handling the full rate of your connection. See this report.
To properly test bandwidth definitively, you need to be in control of and able to isolate all of the variables - test equipment capabilities, local and remote LAN segment load, device under test, upstream devices, other systems currently using the same connection(s), etc.
An easier path is to just neasure your day to day performance (thus establishing a performance baseline) and monitor that for both performance (as observed by the polling and as perceived by your users) and availability. Something simple like a Nagios monitoring tool installation can be used for that. If performance is fine, no need to dig any deeper. If it isn't then start looking into the variables I mentioned above.
Hope this helps.