ukscoobyguy

When I had a 173 via manual diagnosis the code thrown via obdII was p1241 which means there is a problem with the primary turbo. It might be worth simply watching the iacv when the car is started as it should close under vacuum when the engine is idling. (With the engine off it returns to open). Or you could connect the green plugs under the dash, start the engine and see if the iacv and ecv operate as expected. It's 7 second intervals for the ecv and 2 for the iacv.
if the valve operates correctly, then chances are your ecv is open slightly leaking the gases needed to spool up the primary and causing your issues, since the iacv will still be closed until 4000+ rpm If that's the cause you just need to tighten the ecv adjustment rod by loosening the lock nut and turning the rod to the left.

I recently completed this job on my GTB which has an ej206. If I remember rightly they have a 0.6mm mls gasket from the factory. If your going to have the heads machined and it's the first time you should buy the gaskets first so you know how much to take off the heads. I used cosworth gaskets which are 0.78mm so the difference was taken off the heads during machining. It's no good fitting a 1.8mm gasket if your only taking 0.18mm off the heads as you will end up with a low compression engine that is sluggish off boost at best or has timing issues at worst.

Also, if your as boost happy as me , it's well worth fitting a cosworth or ARP head stud conversion if you have the engine apart, I did mine when doing the gaskets and it's been bullet proof so far.

Hope it goes well.