I have tuned about 6 cars already and GM's tune is very much to the edge. Besides a rich fuel mixture (normal from GM) there timing table is very aggressive.
As a matter a fact I spoke to a friend of mine that works at Milford who was involved with the calibrating of that car. He told me the stock timing table is already past MBT spark. This conversation came up after I called him about why my 2010 has KR with the stock tune and good gas.
I did find with the headers that it would take more timing, mostly due to heat being pulled out of the combustion chamber.
Your numbers are right in line from what we have seen as well.