Have you got or can you build a flow bench? That alone would be a good starting point to see where you have room to make improvements in cfm.
On a related note, if you haven't already played with some calculators, check these out!
http://www.wallaceracing.com/runnertorquecalc.php <-- this one is especially handy if you have measurements on the OEM runners -- start from there and see where you can make changes.
https://www.rbracing-rsr.com/runnertorquecalc.html
http://www.bgsoflex.com/intakeln.html
Some very useful tips and maths here too
http://www.team-integra.net/forum/bl...culations.html