Will adding more indexes speed up using RedGate SQL DATA compare? We want to use RedGate's SQL Data Compare to migrate user data from one DB to a DB on a completely different provider. This will be a time-sensitive operation as we do not want our site offline for too long and there will be a lot of data that needs to be moved. We do not want any I/O on the table during this time.

Has anyone using RedGate found that adding additional indexes on some of our comparison criteria columns (i.e. DateCreated, DateModified) will affect the time to perform this? Our initial testing is at about 25-30 min for comparison of the source -> target DBs at the moment.
The question suggests that the tool is being used to migrate data and not schema, which implies that you are referring to SQL Data Compare and not SQL Compare.

In which case, yes, indexes on the comparison key columns will speed on SQL Data Compare.

I've consulted the SQL Data Compare project manager, who provides the following insight: "Usually performance problems with SDC have more to do with the network connection speed to the server and/or the disk speed on the location of the temp files, neither of which are affected by having an index on the comparison key"

SQL Data Compare performance tips are part of the SQL Data Compare documentation.