As far as I know, Collagen starts to break down below 60°C/140F, time play a big role, i.e. to get the same breakdown at 60°C as 80°C you might need 24-48h instead of 3-4h.
The higher the heat the more the meat is contracted and it will get dryer, in a stew that might not be as obvious as in other cooking methods but it should still be the fact. Compare tough cuts of meat cooked sous vide at 55°C-60°C for 24-100h, they get very tender and moist.
So the benefit will be moister meat. I think you must cook the veggies at much higher temperature (80°C-95°C) so you would have to separate the two. If the end result is worth the effort I do not know, I have wanted to try doing a 'stew' with meat cooked sous vide at 55°C - 60°C for 24h+, with the veggies done on the side at 80°C-95°C, normal stew style, to see if it would be better, but I have never tried.
Sources
Hum, I'm sure you or anyone can find enough sources to argue that you need 70-80°C(160-180°F), see e.g. On food and cooking(2004) pg 163 "...Meats with a significant amount of tough connective tissue must be cooked to a minimum of 160-180°F/70-80°C to dissolve their collagen into gelatin. "
The only potentially credible and official (what do you mean by that?) source that potentially can trump On Food and Cooking is Modernist Cuisine, I will quote.
MC 3.78-79 "When meat is cooked at temperatures above 50°C/122°F, a different chemical reaction -- the conversion of collagen into gelatin -- becomes prominent." it continues and says "Cookbooks and some scientific sources commonly claim that collagen converts to gelatine "at" some particular temperature. Usually the cited threshold lies between 60°C and 75°C / 140°F and 167°F. In reality, the hydrolysis of collagen is simply another example of a chemical reaction whose rate varies exponentially with temperature." it goes on describing that the exact temperature have not been measured and that it depends on what type of collagen, e.g. breed, cut and age. Finally it says "Tough meats can be cooked until tender at any temperature above 55°C/130°F (and even a bit below, if you are patient enough)."
Also see lots of sous vide recipes on the net saying that you can cook tough meat at much less temperature than 70°C/160°F and get it very tender. Or try it, I have and it works!!! I have stopped eating primary expensive cuts, flank, shin and brisket tastes much better!
See also SAJ14SAJ reply and Stefanos comment below, as per Baldwin which should be almost as good a source as oFaC and MC
Conclusion
The answer to the question, "how low can you go and still get collagen break-down" I would answer 55°C/130°F or even a bit lower. That is NOT a realistic temperature for cooking a stew, especially not if you will put veggies in it, but as far as collagen breakdown goes that is the lowest temperature for 'stewing meat'.
The only mistake you made was the choice of cut, and maybe the quality of the beef itself. Round (in the UK/AU/NZ topside and silverside) is from the rear end of the animal, and is a working cut. Working cuts have to exert a force, so the muscle must have lots of collagen to distribute the force from the tendon throughout the muscle. Collagen is a tough material which breaks down in the presence of heat and moisture, not just heat, so working cuts make poor roasts or steaks and are better braised or stewed.
The upside to working cuts is that they have a stronger flavor, and the collagen when broken down by moisture turns to gelatin which adds to the flavor and mouth feel. A well-prepared braised piece of beef is as good as a roast any day in my opinion. It's also better value, round and other working cuts are much cheaper than tender cuts.
Your technique as described is perfect, and if you'd chosen a rib, short loin, or sirloin roast (US cuts) it would have come out beautifully. Where you erred was at the store when you chose a working cut for roasting.
Now some people on the forum are probably preparing a rebuttal at this point saying "you can roast round and have it be tender", and they'd be right up to a point. The best quality beef is reared and slaughtered better, so if you bought US Prime round it has a good chance of coming out relatively tender, however it's hard to find. My assessment would be for what you'd find in the average store in the US, which is Choice grade. Choice encompasses something like 75% of beef produced, so has a huge range of quality.
You can tell a lot about meat by touching it. Stick your fingers in it, if they go in easy and the meat springs back then you have a tender cut, if you can't get your fingers in it it's a braising cut. If you stick your fingers in and the meat doesn't spring back its old, so don't buy it.
Best Answer
No, it won't be.
The reaction which has to happen to the collagen doesn't go quicker when the temperature is higher. You have to get it to 68 Celsius and wait for it to happen. If it is at more than 68 Celsius, it won't happen quicker, or better, or anything. You have no advantages.
At the same time, if you have a higher temperature, you have more trouble. First, you are wasting a lot of energy to keep the stew hotter over many hours. Second, its liquid can boil off, getting the wrong consistency. Third, if the bottom of the pot becomes too hot, it will burn on. Fourth, anything non-meat in there can suffer from the heat - vegetables can overcook, starch can burn onto the bottom, anything planty (vegetables, herbs, spices) can get its taste destroyed).
All in all, if you can cook something on a high or a low temperature, choose the low one, it is generally better for everything. Only choose high for foods which have a narrow temperature range which is high.