If the intake valves are heavily gunked up, you could expect a reduction in airflow at wide open throttle, but it will probably have little to no effect on fuel trims.
What you'll need to monitor are mass airflow, absolute pressure at the intake manifold, and intake manifold temperature (or intake air temperature) at wide open throttle.
Do some WOT pulls to get some baseline data, then do the same in a year or two, in very similar temperature and pressure conditions. If mass airflow is lower at the same rpm, temperature, and manifold pressure, then the volumetric efficiency of the engine has gone down, which could be down to dirty valves, or it could be something else, like a clogged catalyst, a dirty air filter, or a dirty MAF sensor.
If the temperature and pressure are a bit different than your baseline pulls, you can use correction factors to account for this. On a turbo engine, it's going to be a lot harder to get really consistent readings.
I've used this method to compare many datalogs that I've taken on my TGDI Subaru from 20,000 to 90,000 km. I didn't notice any long term trend of airflow reduction. All I noticed was that the airflow seemed to be slightly lower when the air filter was dirty.
