It's interesting that your test shows that insert after is slower on jQuery 1.4 than on 1.3 since the release notes for 1.4a says "append, prepend, etc. have been heavily optimized." How about adding append and prepend to the test suite?
Insert element bottom 48 131 169
Insert element on top 48 134 167
Don't know about the release notes, my test says 1.4 is slower than 1.3.2