it would require adding another band to the LTE radio on the phone. A Samsung spokesman confirmed via email that all U.S. units of the Samsaung Galaxy S III are the same across the carriers although the phones are optimized to work on each carrier's own network.
However, just because T-Mobile's version of the Samsung Galaxy S III will be running on the carrier's HSPA+ 42Mbps pipeline doesn't mean it's slow. A member of CNET's crew took off for a drive in San Francisco with the T-Mobile version of the Samsung Galaxy S III in tow and installed Speedtest.net. She was able to get download speeds as high as 16.58Mbps. While not as high as the peak 33Mbps speed she attained with AT&T''s LTE on that carrier's version of the phone, she said that the AT&T service had some dead zones.
Besides data speeds, another issue with T-Mobile's version of the phone is price. The 16GB version of the Samsung Galaxy S III is $279.99 with a 2-year contract or $229.99 for the value plan for which you pay interest. The 32GB version is $50 more for each plan. AT&T is charging $199.99 for the 16GB model, with a signed 2-year contract. AT&T is not offering the 32GB unit although Verizon and Sprint plan on asking $249.99 on contract.