Optimized Way of Scheduling a Differential Backup
First, this assumption is questionable:


  "Cannot use transaction logs as it would affect my load performance -
  datasets are potentially huge with large transactions"


When you do work in the database, your transactions are logged regardless. Full recovery mode just means your logged work sticks around until the next log backup. If non-logged operations are truly important to you, then you need to look at using minimally logged operations like bulk inserts. This is way harder than it sounds - check out the SQL 2008 data loading guide for more details. (For example, you may not be able to do it on a table that already has multiple nonclustered indexes and data in it.)

If it really is a good fit, though, check out this Serverfault post on estimating the size of your differential backup.