IMG failure is a common occurrence as far as I can tell, and can often simulate problems similar to the early stages of HGF (rough running, coolant loss etc). While external leaks are easy to spot, internal ones are much harder.
If you suspect one may be failing, it's best to replace ASAP, as a failed gasket can cause all kinds of expensive problems.
As a matter of interest, what colour is your engine oil?
About 1000 miles in, its the normal color of synthetic oil, you can see the dip-stick through it - not emulsified. But at the rate I'm losing coolant, it probably wouldn't be anyway. My coolant, 1000 in, is still pink.
I would replace IMG if I could find at least one other person who was without symptoms like me (other than disappearing coolant), that was fixed by IMG - because otherwise I feel I am grasping at straws.
I mean, if IMG possible root cause for this lack of symptoms, you would think there would be one other person whom this has happened to, where IMG replacement was the cure. Just 1 !
If not IMG, fire-ring seems likely issue.
Leaning towards the dreaded K-seal option or similar.
My cap holds at the specified pressure, its ok. It seems to consume around town rather than on high speed long runs.
Also, I've had this leak i think for 4 years or so - stable, except where it got worse due to radiator, but revert to prior state with rad replaced. Definitely no coolant around engine or car interior.