The Mitsu 16g is a fairly cheap turbo. Seems like the least painful fix would be to send your ECM back to Hahn (Vector) for a re-flash and set to 91 octane (if that's the best gas you have). If you have 93 available - use it. Find a 16g on E-Bay and have your installer use your compressor side case on the new turbo (for air out size & position). Set the wastegate actuator rod (shorten) to produce 6.5 lbs boost to begin with and put the car on a dyno so you can adjust the boost while monitoring the afr.
Otherwise, for the greatest adjustability and safety you may want to check into getting a Greddy boost controller and an afr gage - look at the PLX DM-5 AFR wideband.
Good luck!
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TEAM SOLSTICE 2008
NASSOA(tm) Founding Member #2
1st 1K #781
DDM Custom Turbo System
DDM Pro-Beam, A-Pillar Pod, PLX gauges
Starlight red I/C pipes and coil cover
GReddy Profec B Boost Controller, HKS BOV
3" Magnaflow cat-free
Werks radiator / Precision billet grilles
Norms vented fenders / GXP Spoiler
Riken Raptor's 255/45/18