I have just completed a 12 week trial with a medium resistance Powerbreathe.
I cycle or row just about every day but do not compete formally so I cannot quote figures of percentage improvement in particular events. Having said that I do push myself very hard whilst riding at the Velodrome and on some club rides which are inevitably very hilly and quite fast.
I only started using the Concept rower for some cross training about 7 months ago so I would expect my performance to improve anyway, that said each week I do a timed one hour row and each week the metres completed increases with what feels like the same effort.
My criteria of success is how I feel under high exercise effort.
As far back as I can remember I felt I ran out of breath long before I ran out of legs, it felt that my breathing had become the limiting factor. I have in the past consulted my doctor about this and had spirometer and exercise ECG tests, both showing for my age of 63 that I am very fit. This breathing limit has been more obvious in the winter months.
I do perceive a change over the 12 week trial in that I no longer reach the point where I begin to worry that, “this time I have gone too far”, with my breathing. By accident not design the trial has been during the Autumn / Winter months.
Bottom line, I think the Powerbreathe has helped with my breathing and I will continue to use it as part of my daily routine.