I don’t recall which marine gear it was, but I recall reading a spec where the manufacturer allows gear shifting at 300 rpm above idle. This is helpful when docking. But this is the first time I’m heard of doing such at 1500 rpm or higher. Perhaps it’s acceptable, but still strange to me.
For hydraulic transmissions, there is no issue what so ever shifting from fwd, to neutral to reverse with a short dwell of 2-3 seconds, and even less if necessary. On most electronic shift controls this dwell is built in and adjustable. These transmission use sintered bronze wet clutches that are forgiving of back driving from the prop, it's not as if you are forcing gear teeth to engage. If you were underway and a small boat cut in front of you, or you noticed a swimmer or diver in the water immediately in front of you (last year while cruising in British Columbia a humpback whale cut directly in front of me as I was exiting a narrow channel aboard a 65 foot vessel, I was doing 4-5 knots and shifted into reverse without any delay to avoid hitting her), you'd need to be able to do this. The whale was bubble feeding along a shear cliff face.
High speed and even full throttle shifting is another matter entirely, however, it too is acceptable under the right circumstances. Yachts that are built to certain classification rules such as ABS, must be able to demonstrate a full throttle shift evolution, from fwd to rev. I've performed these and it's...exciting. That's the exception, most vessel will never be operated in this manner.
For all other "normal" cases, when you make this shift the rpm of course needs to be at idle speed, and with a single lever shift that's unavoidable. In the case of a dual lever control it's possible to shift at any rpm, which could be catastrophic. Several years ago I had a client accidentally grab the shift instead of throttle, pulling it back and shifting out of gear at high rpm, realizing what he did he then instinctively pushed the lever back into place, shifting back into fwd, at high rpm. It destroyed the Velvet Drive tranny, although it was able to be rebuilt.
The stall test, mentioned by another poster, is also very important. I perform this on very sea trial as well. It varies based on engine and vessel, however, generally, I go from fwd at about 6 knots, to neutral, dwell 3 seconds and then shift to reverse with no throttle. Every so often I encounter an engine that will, because of the prop size, gear reduction and idle speed, stall, which is completely unacceptable. Under no circumstances should a diesel engine stall when shifting at a reasonable speed and rpm. The problem can often be resolved with a slight increase in idle speed, of course within the gear manufacturer's requirements.
I have seen pistons exit the sides of engine blocks, but never transmission gears pierce the case, even when shifted to destruction internally.
While every engine should be capable of WOT for 10 minutes (and in some cases no longer depending on the engine rating), it doesn't mean it's a good idea. I go on every sea trial prepared to do this, however, if the engine is in poor condition, with evidence of excessive blow by, or overheating jacket water or wet exhaust components, I usually indicate that the engine needs attention before it can be run in this manner.
I've experienced two catastrophic engine failures on pre-purchase sea trials, in both cases engine surveyors were present. In both cases the engines were old and probably hadn't been run at WOT in a decade or more.