Im on an EPIC Touch running Stock Rooted EL29 ROM/Kernel but i am trying out different leaked modems.
What i have found is the modems can play with speeds.
Example.
Have a friend with all stock Epic Touch.
Sitting in a meeting last week i grabbed his phone and his and mine side by side.
Were set up the same but i had FC15 modem in mine.
3G speed test was extremely slow, but thats a tower / loading issue.
4G speed test on his was 3.6mb down and mine was 7.2 down.
UP his was 1 up and mine was 1.4 up.
Its not the most scientific test but it did prove to me that different modems can make a difference.
Remember the modem controls all the radios, WiFi, GPS, 3G, 4G. The FC 15 modem also fixed my GPS issue that he still has on his EL29 modem.