Let's understand first where the obstruction or restriction is, especially before considering modifications to the pipe. With the pipe empty, remove the stem and draw through both the shank and the stem. Do either seem restricted? Usually, it's the stem, but bits of fluff from pipe cleaners can easily cause a restriction in the shank and can hang on stubbornly -- even when another pipe cleaner seems to pass cleanly. And there's the old eyeball test, depending upon the degree of bend. Can you see clearly through the draft holes in both the stem and stummel?
I hate to ask the obvious, but when you say its draw is tight, is that with the pipe packed or empty? I have found some pipes can be more prone to obstructions by the tobacco based upon the shape of the bowl and/or the placement of the draft hole relative to the bottom. If it's tight only when packed, try putting a pipe cleaner all the way through the stem and into the bowl prior to packing.
Finally, I believe the Derry line are all traditional fishtail stems, but if it's a p-lip, those always feel restricted compared to a fishtail. They still smoke fine, though.