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Intercoolers are a VERY good thing. As you correctly pointed out, they increase the detonation margin. All other stated reasons for having an intercooler are questionable at best.
You made some faulty assumptions, though. The addition of the intercooler does not increase the HP output. That is logical, but incorrect since the increased exhaust backpressure lowers the volumetric efficiency enough to make the HP output just about the same with and without an intercooler. Most people neglect to include that effect in their calcualtions.
The most detonation prone condition is with high IAT (without an intercooler) and with high CHTs and high internal cylinder pressures. It's not at cruise at 17,000 feet as you postulated.... it's on takeoff on a hot day with high CHTs. It is almost impossible to detonate these engines unless the pilot does something inappropriate with the mixture.
The MOST detonation-prone condition will occur at high powers settings at about 40-50dF ROP where internal cylinder pressures are the highest with high IAT. An intercooler significantly mitigates that.
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Walter Atkinson
Advanced Pilot Seminars
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