Are you forgetting the basic principles of a heat exchanger? You're comparing a solid chunk of aluminium of a known size, material and surface area, with a relatively small amount of air flow around it, to a large intercooler, with fins, with a huge amount of airflow going around the exterior of it......

Technically your comment is correct; an intercooler is more efficient. The hidden values in that statement are more like "night and day", as in an intercooler actually makes a different and the plenum makes an immeasurable different to inlet air temps.

If I could be arsed to drag up old physics books i'd calculate it, but for the benefit of example, just look at any big gas heater (like in a workshop). Look at how much energy is required to heat up the air flowing through it.... A bit of heat soak stored in a small mass of metal is really not going to make a noticeable difference.