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Diesel exhaust temps are lower nowadays because of this high amount of EGR (30-50%) used to significantly lower NOx. An older, non-EGR diesel will have much higher exhaust temps (at full load) than a gas engine. Gas engine exhaust temps remain fairly constant because of near stoiciometric mixtures. Diesel engine exhaust varies wildly, as you've stated above. An older (dirty) diesel can be >1200F at full load on its own.
Gas engine CAT: 3-way cat. Converts HC, CO, NOx to "nicer" things. Works whenever the engine is running and is hot enough to do so. Requires stoiciometric.
Diesel engine CAT: 2-way cat. Only oxidizes HC in order to regenerate the DPF. Cannot remove NOx because of lean diesel operation. To elevate exhaust temp at light loads, the ECM closes the VGT to create back pressure, stops all EGR flow and retards the timing to get turbo out temps above 550F. Then it either injects fuel in-cylinder during the exhaust stroke or uses an HC doser on the turbo outlet to spray raw fuel into the CAT (called a DOC, diesel oxidation catalyst). It doses enough HC to maintain 1000-1100F to allow the DPF (attached to the DOC) to regenerate. Regeneration only occurs once every 8-15 hours of operation, otherwise the CAT is basically doing NOTHING but going along for the ride.
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