One of my associates did this about a year ago.  The import worked but it became impossible to add anything further until we upgraded the plan.  It was just over 1100 records, which might be within a threshold, a much larger upload might have different results.

I imported over 2k records. AT is letting me 'add' more records, but I see the 'Over Limit 30-day flag' (top right of the navbar). That's cool. In this case, I only need to work with a few hundreds records so I have time to edit.

It would be good (for me :-)) if AT offered a 5k plan for smaller jobs; but that probably does not fit in their subscription model very well.