The problem sounds very much like cam timing, mate of mine had a bt14 and bd16 in his and when I drove it I thought I had pulled away in 3 rd or left the handbrake on as it just so gutless. Also if you didn't hit the red line before changing gear it would drop out of boost and felt gutless again before it started to go again.

This was all fixed when he had the cam timing reset and then it drove quite nic off boost and came on boost a lot quicker.

Im sure I read somewhere you have none standard cams in but just reread you post and no mention of cams !