It's awfully easy to try and predict how dominant a "good" runner would be in triathlons, but usually they just end up being good triathletes as well. Ryan Bolton (with 2:49 run at IM USA) was a good runner (12th at '95 NCAA XC while at Wyoming, I believe). Greg Whiteley (sub 13:30 5000m) never became more than a decent pro triathlete. If one were to speculate on what they thought either of these guys should do for the run in a tri, I suspect they would estimate much better times than they actually achieve.
My own experience is that a good runner loses a lot of time on the tri run compared to an open race of the same distance. I regularly slog through triathlon/duathlon runs at less than tempo pace and still have one of the fastest splits. For me it seems that I lose economy and smoothness when I try and run off the bike. It's more like the feeling of trying to hit race pace 15 miles into a good long run. It doesn't matter how easy 6:30 pace is for a long run, it is a whole different world when you have just come off of 4.5-5 HARD hours on the bike.
Guys like Mark Allen and Peter Reid were never really fast open races, but unlike the best runners-turned-triathletes they could come pretty near their open race bests in a triathlon run. Allen ran 2:40 in IM Hawaii (the course record), but he never was much faster(I don't believe he broke 2:35) in an open marathon, even when taking time to focus just on the marathon for a year or so. (Reid's ~2:35 at IM Switzerland a few years ago was on a short course - I don't believe he's run under 2:40 for 26.2 mi in an IM).
It's interesting to see how top-level single sport athletes do when they take up multisport. You can get pretty competitive fairly quickly, but a lot of that is lack of depth in many races. To get to the top, though, you have to maximize your performance in all three disciplines. If you are putting enough effort into running to approach your potential there, cycling and swimming will suffer to the point you won't be competitive in triathlons.
On a different but related note, discussions here of running depth, Gallowalking, etc. often lament the loss of the 2:30-3:00 marathon segment of the running population. I would suggest that maybe a lot of the people who would have been the "serious recreational" runners of the 70's and 80's have turned to tri's in the 90's and 00's. The dedication to training, racing, and the sport of tris in general from the age-groupers is much like the dedication to running that gave us lots of sub-3:00 times from average runners in the past. I think the slow conversion of races from competitions to social gatherings has directed a lot of people with competitive ambitions to multisport, which has much of the feel of the 80's road race scene (from a competitiveness point of view).