It can adjust up to a point, but for heavy mods it will need a remap. Even with light mods like pipes and air kit, a remap wouldn't be a bad idea, here is why.
If the ECM has an adjustment range of 6 in either direction, then it's good from -6 to +6, if your mods put it near the edge, say it's at +5 now at x RPM, if you go into an area that the O2 sensor tells the ECM to adjust it up a couple points (due to elevation or whatever), you no longer have a couple points to go up, and you'll start running lean....or rich, whatever the case may be. If you remap you've now got your ECM back to a 0 point (or close to it) and it once again has a range of 6 in either direction.