The cold start mechanism involves enrichment via the injectors when the ECU temperature sensor signals cold to the ECU and extra air from the throttle by-pass regulator being open provides cold fueling mixture - idle speed is raised and the engine runs like it on "choke".
If the temp sensor is doing its job, the enrichment tails off as the regulator shuts down until normal running temp is achieved.
If the temp sensor is still signaling "cold" when the regulator has closed the engine will still be running rich at lower rpm.
Increased fuel pressure may not have much effect when the engine is in "cold" mode, but if still high when warmed up, each injector pulse will release correspondingly more fuel because of the extra pressure behind it...
I won't condone thrashing an engine when cold, but a small test on one occasion isn't going to do any harm. Also, do the ECU self diagnosis and see whether anything critical is flashed up.