if you are just widening you aren't messing with port timing just port area- do 2mm on each side. if you want to mess with port timing lower your intake window by 1mm and raise your exhaust window by 1mm. as long as your porting is consistant between the intake and exhaust (lower intake/raise exhaust) then you are golden.
for the 15mm carb i would put a decent puch pipe on that mounted on the other side versus a crappy biturbo. and yeah make sure that intake is 15 cause if it is not then all your efforts are wasted. good luck.