Stay with regular.. 89 is not regular is it? That octane will change by region and their altitude. For example in the great state of Utah regular is 85 Octane.
With that long tube header you should be thinking of a tune though. people have observed that they can get some what lean under WOT and a long tube header and free flowing exhaust.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)