In real terms, how much effect does a bad fillfactor have on performance?
This is only a partial answer to your question, because you don't provide enough information on how the database will be used.  I also think this question may be more appropriate on the database administrators site.

For instance, if the data is loaded nightly and the database is only used during the day for querying, then you want the fill factor to be 100%.  That will be the most efficient approach for querying, minimizing the number of pages.

Page splitting in indexes occurs when a new record lands on a page that is full.  The page is split into two pages, which are then, roughly, 50% full.  Once the page is split, there is more space for those values.

If the data is being updated with multiple updates or inserts per second, then page splitting is unnecessary overhead.  However, on indexes, this will probably happen the first time a new record lands on an index page.

Note that page splitting is going to happen anyway, once pages are filled.  This would happen anyway, regardless of the fill factor.  The major question is "start-up" time, because initially basically all inserts/updates will result in page splits.  But this should quickly dampen.

The overhead also depends on the characteristics of the implementation.  If the indexes fit into memory, or if your disk system is solid-state disk, then the overhead might not be noticeable at all.

My guess is that this is not the most important problem, unless consistency of performance when the new database first goes online is paramount.