Alright, first let me say that I do not live in a hspa+ area. But I used to (Ft. Lauderdale) and I know what it was like to get 4-6 mbps downloads.
Where I live now I get excited if I broke the 2 mbps threshold. But supposedly AT&T says I should be getting speeds closer to 3-4 mbps down in my area.
So I have been doing a lot of tweaking to see if I can get better speeds and I believe I have found it.
Anyone can do this, even a noob.... But for me I use SU file manager which is similar to root explorer and works the same.
You simply edit your build.prop file and change the value to those below. I usually copy my current build.prop to my sd card, edit that one then copy and paste to /system.
Don't forget to reboot - TWICE for network changes to stick!
As always neither I or anyone else is responsible if you mess up your phone. You should always have a current nand backup for anytime you tweak your phone. Also a build.prop backup copy is a plus.
UPDATE: If you are missing any of these in your build.prop, you must add them. See last pic below for how mine looks.
What will this do? Well for me it more then doubled my network speed. I have attached a speedtest.net screen shot to show the difference for me though I should note that I use Root Metrics coverage map for testing purposes.
Will it work for you? Most likely.
Will it hurt to try? No.
Will you be stoked if it does? Hell yeah.
Will the world end if it doesn't? Nope.
Sent from my Samsung Infuse 4G - 1.6ghz