It's pretty accurate, of course it is my app so i will say that.
If you are unsure, turn on wifi sharing with just your cellular connection active and test with another device.
I just tested and got these results on my HD7
Shared connection 3g - 2 bars via speedtest.net:
0.60 down and 0.09 up
0.73 down and 0.13 up
Non shared direct connection 3g - 2 bars via BandWidth:
1.25 down 0.53 up
0.98 down 0.60 up
Considering the loss you get with sharing your connection, i'd say its pretty accurate
More importantly for the purpose of this thread, as long as you use 1 source as your base for testing, you will see correct results. So if all of the tests are done with the website speedtest then you will get accurate data in which to test your theory of 4g limitations on, same can be said if all tests are done with wifi sharing, or all with BandWidth. As long as your are consistent with your method of testing. You will get results.