Date: November 12, 2012 09:41PM
Here's a little bit of thought.
The Book of Mormon: Helaman 3:8 And it came to pass that they did multiply and spread, and did go forth from the land southward to the land northward, and did spread insomuch that they began to cover the face of the whole earth, from the sea south to the sea north, from the sea west to the sea east.
Massive cities destroyed and built: For example, 3 nephi 8-10 details the destruction - by Jesus - of at least 13 cities. That's not that limited of a geography. Furthermore, after Jesus appears at their temple in bountiful, they take a few days to gather a small minority of people from the lands around them. And even further, in 4 nepho, it takes them years to go out and proselytize the rest of the people to form their utopian society that eventually "they were spread upon all the face of the land" (v 23).
The D&C: Doctrine and Covenants (which specifically identifies North American Natives as Lamanites) and even Joseph Smith himself. If one is to accept any variation of this 'limited geography theory', one must completely disregard the LDS church's current and past declarations on the matter and basically stand in direct opposition to current and past church teachings.
Weapons Proliferation: Lehi and family brought with them significant metallurgy and weapons skills. There is zero evidence for this. Such weaponry would have been adopted by Ancient Americans and proliferated well beyond supposed jewish newcomers.
Zelph: Can't both be in Mesoamerica and in the Midwest US. See http://www.mormonthink.com/zelph.htm
Furthermore, by adopting the LGT the LDS Church is painting itself into a corner that might eventually force it to disappear or at least wholly re-invent itself.
LGT->elimates Flood->elimates young earth and Adam & Eve
LGT->limits Lamanites->diminishes Israel/covenant->muddies gathering & 2nd Coming doctrines
And these together diminish the doctrine of Christ.