Dont waste your time with a walbro, the stock pump will be fine for the stock turbo, unless the stock pump is faulty.
When my car had the stock r33 smic, i noticed a difference in performance when upgrading to a fmic. It did have a tiny bit less throttle response, but it seem to raise the boost and also received alot of cold dense air.
Id say get a tune, he could probaly adjust afr's again and make the car more economical. Dont worry just about power, there's a saving just by having a clean tune. Whats $200 when you might spend an extra $20 on fuel a week from a rich tune.
Tune it if you plan not to mod it anytime soon.
Cheers
Daniel