Timequest and external intefaces

Your spot on in your thinking. I do that all the time. (Note that you only showed the Summary reports, whereby if your worst case path isn't on one of these I/O, then the summary slacks would be the same). If you do a report_timing on the paths, you should see line items that are labeled as iExt or oExt(i.e. the external delays you entered) and you should see they are part of the slack calculation. So if your changing the numbers, either this number isn't changing, or something else is changing in tandem(which doesn't really make sense). Anyway, do the original report_timing on the paths in question and write it out to a .txt file or do a screen shot. Then modify the .sdc to this newer version, reset the design and re-run the report_timing, and then compare the two. That should point to what's going wrong.