That is to improve speeds on the data side. It shouldnt have anything to do with signal strength or quality.
Also comparing the EVO 4G to 3D for signal strenght is not the same. As i mentioned before there are differences in manufacturing, antenna placement and other things that make a comparison not so accurate.
There are always small amounts of signal interpertation differences in all devices. Especially between manufacturers.
Low signal is just what it is low signal. If your 4G gave you 1 bar and your 3D gives you no bars, but you still have signal, then that might not mean anything.
If one devices gives full signal and the other has 1 bar then yeah something is wrong.
At my desk in my office my 4G gave me 1 bar, my EPIC gave me 2--3 bars, and my 3D gives me 0-1 bar. The fact i still ahve signal is good enough for me. If i had no service then i would complain.
To gar a fair and acurate comparison at least have identical devices. Find someone else with a 3D and set them side by side for comparison.