I am using the ST18a on AT&T and getting the full benefit of HSDPA 7.2 speeds (same as iPhone 4). Wifi Hotspot and USB tether both work too.
You can go HERE
for carrier APN settings. (see very bottom of thread for correct ones).
Here are some data speed results vs. my wife's Neo using speedtest.net app.
34 tests to 18 servers each for Neo and Ray (same servers). Tests were performed one after the other (alternating Ray and Neo) in an AT&T HSPA+ w/enhanced backhaul coverage area between 8PM to 9PM.
PHONE Download Upload Ping
Xperia Ray 1,735 868 402
Xperia Neo 1,722 875 420
Xperia Ray 2,382 1,084 277
Xperia Neo 2,264 771 185