n75 or mbc

to cut a long story short i got a k04 ibiza, got a n75j fitted, overboost 14psi tailling off to 10 or 11, i'm not happy with this so i bought a mbc to try, swapped it for the n75 but keeping the valve plugged in i get overboost 23psi tails off to about 16psi but hit limp mode sometimes, if i...