There's a good benchmarking discussion of the new Air 19 Array and Vector functions, insertAt and removeAt in this thread:
@bwhiting, @NagRock, and @booster: The real performance gain cannot be measured in a tight loop with immediate timing results. The real performance gain comes from sustained execution over main frames. Using Scout to check the performance of our game, we found many occasions where the killer issue was the garbage collector. We went through the Starling code several versions ago, for example, and replaced the use of splice and String concatenation everywhere we could.
We also found that the native Adobe JSON parser is faster than actionjson (writen in AS3) but over time, the blow-back of the native JSON parser's internal String allocations comes back to bite you. actionjson results in over-all better performance over time.
To measure the real performance gain of insertAt and removeAt, write an app that does 10,000 operations per frame and runs forever. Then look at the per-frame execution and garbage collection time in Scout. I believe you'll see a significant over-all improvement.