GMAil to SmarterMail enterprise email migration

Have ~100 users in corp. GMail account.  Pareto's 80/20 principal applies as usual, where 20% of the users have 80% of storage consumption.  Of course the largest mail boxes are key people and are as full as 15GB's.

Have SmarterTools SmarterMail enterprise v11x (an Exchange clone) and will migrate everyone over to this private server using a different domain address for the new server (not moving the old email domain right away.)

SmarterMail has an import tool for each user.  This looks like a good tool, but several things come to mind.  1) If a user account is as big as 15GB's, then no matter how great the tool is, that's still going to be 15GB's at internet speed and 2) Users have custom folders on GMail; if email migration just grabs and dumps from GMail to one folder in SmarterMail, will have many unhappy campers.

Before we start and find a long miserable migration that we can't stop (in for a penny, in for a pound) is there a way to mitigate these two issues?  Is there an email migration software that can chunk the transfers vs. "all in" of SmarterTools and can it create folders and organize on import?  Prefer a Win software but if Linux, we'll take it...