Regarding Stahl, I remember seeing an article or interview about 25 years ago stating that his best 10K was about 30 minutes. I've tried to find his PRs for various distances, and have looked at lists of top times by Swedish runners, and all that I can determine with much assurance is that Stahl never ran a road 10k under 29:40 and never ran a sub-1:04 half-marathon. Considering that he barely lost out on a bronze medal at the 1983 world championship marathon in Helsinki, getting passed at the very end by the doper Cierpinski (2:10:37 to 2:10:38, with De Castella winning in 2:10:03), that's pretty remarkable. In later years, his half-marathon pace was barely faster than his marathon pace (after the age of 45, he ran a number of marathons between a low 2:15 and 2:20, but never managed to break 1:07 for a half-marathon during that time, despite a number of tries). At the age of 49, he ran 2:19+ for a marathon, but only 31:21 for 10,000 on the track. At the age of fifty, he ran 2:20+ and 31:42 on the track.
I ran against Stahl in the '80s, and had a chance to talk to him about his running career. He said that, up until his early 30s, his athletic focus was on orienteering, which was (and, as far as I know, still is) big in Sweden. That probably gave him a tremendous amount of strength and endurance when he switched over to road-racing, but not much top-end speed. He also was one of the very few world-class marathoners with a legitimate job outside of running, and I think that his daily circumstances limited his training largely to distance runs and hill workouts in the evening after work. He was a beast of a runner; I remember hearing his footsteps crashing the ground when he came up from behind late in a marathon. It was like being chased by a dinosaur.
Regarding Beardsley, I have a vague recollection that he easily won a 10k a few weeks before Boston 1982 in a low 29 (for some reason, I'm thinking something like a 29:16). Obviously, he was capable of faster.
As you apparently already discovered, some of the reports about Ian Thompson's slowfootedness have been exaggerated. I think he ran 29:33 five years after his best marathon, and 3:51 for 1,500 five years before his best marathon, so he clearly had more speed than some have suggested when he won the 1974 Commonwealth Games marathon.