I did use the original (well, stock replacement) for the feed on mine. That said, on my drive from San Diego to Austin, the pump was getting loud and warm to the touch at the end of each ~500 mile day. Now, it's been fine around town for the last 2 years, and the inline pump is mounted as close to the tank as possible and right at level with the bottom of the tank. But I do believe it's on it's way out and I plan on re-doing the feed with the recommended larger feed line.
In all, I think it's probably pretty important to run the correct size line. It didn't cause any trouble at all until I made the cross country drive. It made it, and sounds fine when I drive it around but I suspect it adds just enough strain to prematurely kill the pump.
Also of note, I didn't see anywhere in the instructions where it talks about using the PWM feature with FiTech's inline kit, so I had it "off" at 74. When I talked with FiTech's tech support while on my cross country trek, they said you can adjust PWM down to "as LOW as 60" with their pump. The limitation would be making sure you're not going lean on AFR's due to the pump slowing down, but had I known this I believe that would have helped take a lot of strain off the pump as well.