Got my own answer!!!
List of cities with HSPA and HSPA+ refarmed to the 1900 MHz:
http://androidandme.com/2012/12/car...-new-metro-areas-to-4g-hspa-on-1900-mhz-band/
This list confirms San Diego to be refarmed 1900 MHz HSPA+:
http://androidandme.com/2012/12/car...or-att-and-unlocked-phones-in-23-metro-areas/
Which should mean that HSPA+ is also available for unlocked AT&T iPhones, not just HSPA.
HERE'S a small part of article:
"...T-Mobile has been moving pretty fast and today they announced that 23 metro areas, reaching over 100 million people, now have the enhanced network. This means that unlocked AT&T phones and some global phones can now experience 4G HSPA+ 42 Mbps speeds on T-Mobile’s network..."
CONTINUES TO PART WE CARE ABOUT:
"...Internal T-Mobile tests of unlocked iPhone 4S devices running over 4G HSPA+ on their 1900 MHz network recorded on average 70% faster download speeds than iPhone 4S devices on AT&T’s network."