Hmmm, interesting. I am running DiF for 6 1st levels right now and they are finding the fighting pretty easy. They are relatively high powered, so I'm sure this varies from campaign to campaign.
Without knowing how powered up your guys are, I can only suggest you take a look at Terror in Freeport which is intended for levels 2-5. It might be easier to modify *it* quickly to take into account the few significant discoveries of DiF, or, if you really want to run DiF first, it will give you a good idea of what challenges are appropriate for 2-5 level characters.
One thing you've probably already noticed is that DiF is very short. THis means you need filler material if you are going to run the Freeport series alone, but in your case its great -- you only have to rebalance a couple major encounters before you've moved on to Terror in Freeport.
One specific thing I would pump up is the Orc Ship. Most groups seem to want to take these guys on, and a party of 3rds would probably carve right through them as written. Its fine I think from a game-play perspective if the party just rolls the first two major encounters, the Orc Ship and the final battle are the only things that require careful tuning.
Hope this is useful, even if I didnt directly answer the question. ;-)
