larger headers will be less responsive due to slower gas velocity till you come on boost then will flow more....

By twin scroll do you mean split pulse???

Is the turbo a garret gt 40r??? would think that would be pretty laggy on a wrx???

Is it the 88mm wheel or the 94????

cause I have a gt 4202 for the rx3 and it's pretty bloody big.... has 102 comp wheel in it..