It would be difficult for anyone to give you an estimate on the difference been a 10 ft diameter Edward and a 15 ft. However, when we are talking about shy bearer, we are talking about really low production compared to neighboring trees of other varieties. For example, one of the Edward trees I harvest has maybe around 50 pounds of fruit on it at around 30+ ft tall. A Zill tree next to it has literally hundreds and hundreds of pounds of fruit and is roughly the same dimensions.
I also happen to have a small well-branched tree around 4 feet tall that is holding exactly 4 fruits to maturity. A medium producing cultivar of similar size may have more than twice as many fruits in a normal year, while a productive variety like Pickering or Glenn may have even more than that.
As far as consistency, I would actually say Edward is one of the most consistent mangoes in terms of actually producing a crop every year. This is probably due to the fact that it normally has two blooms....one in November and another in January. So while other cultivars may wind up producing more fruit most years, they could 'take a year off', which Edward trees rarely seem to do if at all. This trait also results in a somewhat extended ripening season for it I'll typically get them as early as May and still be getting a few ones as late as July.
The beauty of the Edward is in its sumptuous flavor. Its a true classic. If you grow it, and look at the production, and wonder why its still there, your suddenly reminded why when you eat one again.