Ok, it'* hard to say, because nobody has really modded an L27 on this Forum aside from the basic bolt-on'*.
My suggestion based on what I've learned on the Series 1 in the last couple years:
1. Fix the EM cracks, reinstall with gaskets. FREE if you have a friend with a welder
2. Port-match the LIM ports to the cylinder heads (don't match to the gaskets). FREE
4. GOOD CAI. Shield yours very well. Nearly FREE
5. High-flow Cat. It'll do more for you than that muffler, and you'll need it for this:
6. 1.8 YT rockers. Keep stock pushrods and springs.
Much of your car'* performance is your tires and you. You could shave 1-2 tenths in the car'* present form depending on those two things.
If you can shave a tenth in it'* present form, the above list might get you to 15.6 to 15.7. VERY noticeable in the seat of the pants.
Not including gaskets, you'll be in $550 for the PCM and rockers.