I was reading an old issues of Performance Bikes when they reviewed the 990SD and one thing they said to do was to set the valves to the top end of their range (largest clearance) as this stops the engine loosing power as it gets hot.

Is that BS?

I'm not aware of any historical evidence of the LC8 engine ending with no clearance when hot if the cold clearances are at the lower tolerance, but it's always good practice to set them to the upper tolerance when adjusting - shimmed clearances only ever reduce over time due to valve seat wear and valve stem stretch so if you go to the top end of the clearance range, you'll get the longest time before they have to be changed again, and potentially go under-size which could lead to valve seat burning issues.

As folk of my Dad's era say, "a noisy tappet is a happy tappet"...

Is that the same article that says there are no gains to be had by removing the stock airbox in favor of a Rottweiler or motohooligan box?

It was Sept 2017, and a Mod Squad feature..

The engine the guy (Dave Wood) does say not to remove the airbox but instead to just fit a free flowing race filter.

He says that the fueling isn't the problem (for the jerky low speed riding) as it's mainly the ignition curve, so they adjust that but also adjust fueling to match.

He also states that on the race bikes they "play around with the valve clearances", and that by opening them up they consistently gain more power. With standard clearances the engine would lose power the hotter they got, so towards the end of a race. Larger clearances gave more consistent output.
If that is the case then I doubt the effect would be noticeable for street riding.