I would agree that the 1:42 is a poor predictor of his success at the 1500 and an even poorer predictor of success at 5k.
I've had athletes that have run 1:48 for 800 and 2:25 for 1k but really struggled from the 1500 on up. I would suggest that his unique abilities suit the 800 and slightly longer distances (1k-1500m possbly) but don't indicate any great times in the 5k or longer distances.
There was a previous post that suggested 30:24 for the 10k or something close to that. I would completely agree. I have athletes that can run 30:40 but can't break 4:04 for the 1500. Leg speed is part of the equation but at the same time I would suggest that you have to keep in mind each organisms unique ability to combine speed and endurance.
I would predict a 14:xx low time for Nick on best day ever scenarios. Regardless of a change in training, certain athletes can only do certain things. He might train for years at longer intervals including an increase in mileage and some excellent training at Tempo pace and have minimal success at the longer distances. Bumping mileage doesn't always translate into faster times at longer distances...........