A mate used to run a high flowed 25 turbs on his 20 and was seeing full boost way before that. Off boost driving was pretty poor but barable. A tune would help but as said it depends who high flowed it and what bearings are used, size of wheels etc etc. Tune better fuel pump and bigger injectors and decent boost control will see gains everywhere and you will be happy. The tuner will probably play with the timing anyway.