No it won't. I have an unlocked, non-carrier, 855.
HSPA+ speeds only (as expected and previously known by me).
In my area the AT&T HSPA+ speed varies (depending on what part of town I"m in) from 10 ~ 13 MPS.
Not strange at all.
LTE 700 Mhz "band" by itself doesn't mean much except AT&T uses somewhere between 700 Mhz and 800 Mhz.
The "sub-band" tells the story. AT&T uses 700 Mhz, band 17. I can tell you for a fact that the 855 (at least mine) does not have band 17.
Bands 12, 17, 28 and 44 are all sub-bands in the 700 Mhz LTE "range".