Here are some stats for your perusal.
Total win rate for AA......3.72PTBB per hand
Total win rate for KK.......6.00 PTBB per hand
AA win rate when limp reraising......19.25PTBB per hand
KK win rate when limp reraising.......9.69PTBB per hand
AA when limp..pot not raised........5.76PTBB per hand
KK when limp..pot not raised.........2.06PTBB per hand
AA when I open raised...............-4.31PTBB per hand
KK when I open raised................3.96 PTBB per hand.
You see how the numbers are mostly higher when I didnt raise 1st? One of the reasons for this is that when you raise and get a caller or 2, and there is heavy action on a flop like 974, they are liable to have a set and its going to be expensive to find out. But if THEY raised 1st, theres little chance they have a set on that board.
I only have 7000 hands in this database so some of these numbers will converge over time (especially the one where I am -4.31 when open raising with AA. I got busted by a set and that takes a big toll on the win rate when the number of hands is limited.