From what I've seen it does cause a problem, the top of the FB really should be the same as the BP for the heat to move in smoothly as possible. I've seen several people do mods on factory offsets and the has been very helpful to them. Can you add to the bottom instead?
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........
If I'm not mistaken (and I have been before) your FB won't yield enough CFM's of hot air/radiant heat to heat all the total cubic inches of CC to get up to the temp you want to cook at. Adding to the bottom would be the very best direction to go by far.
And on the eighth day God created barbecue …. because he DOES love us and he wants us to be happy.
Current smokers: Egor (trailered RF) and Easybake (tabletop pellet drive)