Is there a reasonable upper limit for the brightness of a lamp?

If yes, then it certainly depends on the application. The breathtaking 20 000 effective Lumen of the model 623 are surely too much for normal applications, but exactly that makes it ideal for mining vehicles, van carriers and harbour cranes. This lamp is even used for illuminating sports stadia as modules on floodlighting masts. Much more light than this is really not required!