True... AT&T most likely puts SIM cards that are provisioned for Go Phones into a throttle pool.I see what you mean now but regardless of how it's accomplished go phone prepaid does not get HSPA+/4G speed.
There is no APN setting that can get around a throttled SIM card.
This still can't override the radio.
So you can still get a 4G (HSPA+) signal on a go phone SIM. It will just be throttled.