Shouldn't this be a fairly well known and documented issue? It's been going on here for quite a while and the International Association of Fire Chiefs have been studying this issue for over a year. They released an updated "Portable Radio Best Practices presentation" earlier this month based on this research.
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http://www.iafc.org/displaycommon.cfm?an=1&subarticlenbr=719
One interesting line in the presentation was "Radio systems that are designed and constructed without consideration of fire department requirements frequently do not meet the needs of fire and rescue users."
Also released this month was the National Telecommunications & Information Administration report, "Intelligibility of Selected Radio Systems in the Presence of Fireground Noise: Test Plan and Results". If I read the results correctly, it shows that analog FM exceeds P25 in readability on about half the tests and is equal to it on the others.