The Samsung Galaxy S II may not be available from any US carriers, but those who have imported it are able to use it on AT&T's "4G" HSPA+ network. If you are using the Galaxy S II on AT&T however, there may be some hangups in its ability to work on their network, resulting in hampered data speeds. Well where there's an Android phone as awesome as the SGSII, there are devs to make it more awesome. XDA member locust43 has a fix to get your shiny European phone blazing along at full speed.
The Galaxy S II is almost uncontestedly the fastest phone out there hardware wise, and with HSPA+ radios, its data speeds aren't anything to sneeze at either (unless of course you have LTE). If you're using the phone in the states though on AT&T's network, things might not be meshing exactly right. Do to some differences in network settings the phone isn't able to reach its full potential right out of the box.
This is a more hands-on work around, but is not overly complicated. It may be wise to backup your phone with nandroid, or at the least write down the old APN settings that you will be changing, as their have been some mixed results in the forum. What you will basically be doing is modifying the APN (access point name) your GSII uses to connect to AT&T's network to make sure it is getting full HSPA+ speeds.
Be warned that things get a bit more complicated for anyone using anything but the AT&T 4G 2GB plan, if you are lucky enough to have unlimited data still, or simply haven't been changed to a 4G plan according to AT&T a bit of extra trickery, or talking to someone at an AT&T store, may be necessary. If you're brave and knowledgable enough, and want every little speed boost you can get your hands on, backup, give it a try and let us know if you're seeing any improvement.