if u get a s-afc / e-manage and then get tune with ur current mods it wont make that much of a difference... plus u probably have to get retune once u get it boosted..
most skylines run rich once u start adding mods so u dont have to worry about ur car leaning out.... so im with hippy, get ur bolt on stuff ( dump pipe, ebc, fmic or wateva ) then the s-afc / e-manage..