It strikes me that those who say there isn't significant difference these days are talking about part 25 or larger military aircraft. Certainly where I earn my living (part 23 or smaller) these differences are as important as John describes (I recall putting a +30°C perf write-down on a twin 2 years ago). I'm guessing because down here in the weeds we can't afford the sort of production / QC technology that has become essential for the likes of Boeing / Airbus / BAE(?) over the last 10 years or so.
G