first off....what's a boost timer? lol
and second off....you basically want to 'tune' your car by adjusting the apexi fuel controller to keep the AF gauge in the middle (stoich). Like, if it's rich under 3500 rpms, adjust to lean out a bit -%, if it's lean up top, adjust to add fuel +%. What type of fuel controller is this? an s-afc, regualr afc, etc.
what kind of turbo is it upgraded with? i'm sure turbo larger than stock should be efficient enough to the 16 psi range at least, depending on how much larger it is. I don't really know mitsu. turbos so i'm just guessing.
do you have an egt guage? or aftermarket boost gauge? i know i read they were the stock gauges, but most ppl will get an aftermarket boost gauge with a boost controller. at least ppl who do things right will.
hope this helps a little