as you havnt done any other mods you won't feel any big difference. and the ga15 TB is a direct fit! Hassel free apparantley. not alot of people have swaped over to the commodore TB as far as i know.
the standard inj in your micra are fine if you just intend to swap over the TB's, if you go a little further in getting more power in a NA setup then you would start looking at injectors, primera ones are another direct fit apparantley.
what is it your after exactley?