[Prisen]

The fastest implementations of HSPA-plus are actually coming close to the theoretical limits of how much data can be transferred on a given frequency band, but they have some big drawbacks compared to LTE.

The one that will be most noticeable at first is latency, which is at most half that of HSPA (the requirement is 20ms for LTE). The second is that LTE will be able to use bigger frequency bands than HSPA, eventually reaching a 100 MHz band.

Even ignoring that, AT&T's current HSPA-plus network is nowhere near as fast as HSPA-plus can be. They would need to upgrade it to get close to the raw speed of LTE.