I had the same problem on my 7mgte turns out the manifold studs were all hand tight the maifold brace was not attatched and the gasket was destroyed ad the manifold was cracked. Check all of this ad alll your vacume lines they all make a big difference

wow. sound advice would be to get a new mechanic. sounds like hes taking you for a drive. you went in for an exhaust leak and came out with 2k in parts.. now hes suggesting more. sounds like someone is trying to cover up some foul work. more information on this please.

Under Drive Pulley
 

I need to clarify that it was my under drive pulley that was installed at my request so the shop cannot be blamed for that. Another mechanic suggested that many Japanese cars have their crank pulleys balanced with the crank so that is likely why the new pulley didn't work.
But it really ran like shit!
I highly recommend not changing the crank pulley unless you have it balanced with the crank - just say NO!

The boost still climbs slowly and won't go above 7 or 8 lbs. I'm thinking maybe the cat is plugged, although it is only a month old, (but so were the plugs and they had to be replaced).

Back to Split Fire today to check this out and hopefully finally have the car tuned on the Dyno. I have a time attack at Mosport this Saturday.

Exhaust leak
 

I need to mention as well that Split Fire fixed my under-hood exhaust leak.
I will ask him today what the problem actually was.

Over Boost
 

Just got back from the shop - 3 hr highway drive.
She's back baby!!!
It was the cat. Cleaned it out an voila - all the boost i can stand.
Only problem is for some reason my under hood manual boost controller will not lower the boost so i have full boost all the time - it will go to 15 - 17 and then cuts out. I'm getting a anti boost cut unit or what ever you call it on Monday but that doesn't help me this Saturday at the time attack at Mosport.

Does anyone know of a trick of some kind to lower the boost short term??? I cannot floor the gas because it hits the cut off. It's quite tricky to get maximum boost while staying just under the cut off point.
If anyone could help with this it would be really appriciated.

And the exhaust leak was a loose bolt on the downpipe.