I don't know about XCSoar, but with WinPilot, the terrain data are
used to calculate final glides, and knowing there is a mountain
between your goal and you can make lots of difference! So I wouldn't
advise to turn the terrain data off. What I do is changing the
settings so that every relief below say 2000 ft doesn't show up on the
screen. I still get roads and streams. If XCSoar can do the same, I
suggest you experiment with the settings for relief.