Would this hole/slit in the joining between the throttle body and the manifold be causing a loss in power for my rex? When I took the car in for a service recently they didn't say anything, although I didn't tell them about it.

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I doubt its a gap, just a wavy gasket.
If it is a gap, idle would be affected aswell.

start the motor and spray some aerostart around the gap and see if the idle changes at all.
If it does, it does leak, and it will leak air under boost situations.

Just crank the 4 bolts up.

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Balls.

the mating faces look a bit off from the image, like the gaskets worn off or dirt has gotten into it.

Don't blindly crank the bolts, diagnose whether it's actually a problem first. i.e. see post above.

The idle is actually a bit random. Sometimes the revs will just drop and rise again over and over whilst idling, and when it doesn't do that the idling rpm is never the same.

I'll give the aerostart a try though, thanks