doesn't sound like symptoms of a failing turbo

does sound like a failing air flow meter though

is your mechanic familiar with wrx's? because thats a pretty bloody common cause for the old hesitation coming onto boost at around 3000rpm

seeing as the latter is only about $200 and takes about 2minutes to fit i would try that 1st (cheaper if you get one from mao at evolution R)

hope this helps

budget for a new bottom end if you go installing a vf34 and smashing more boost