I have found that winning constantly does give diminishing returns. Why so? I couldn't answer, but I have found that losing a game here and there does increase the amount of Points you earn. I'm currently sitting in Division I of Gold, and I'm making about 12 Points per win. Of course the last Division of each League seems to give the lowest amount of Points per win as well.
Since the PBE doesn't have a huge number of people in similar Divisions and Leagues to choose from we have to face higher and lower opponents. The modifiers to the Point System may be what are causing the low point gain. So try to play on higher population times and see if that changes it.
Players that have been losing alot of games tend to get a large amount of points when they finally do win a game. AKA your Platinum Player who gained a bunch.
Of course everyone's experiences with the system will differ, depending on how skilled they actually are (and Afker's, feeders, etc). Some people did really good in the old ELO system because they got lucky or had a good duo partner. Since the advent of the new items it's harder for some champions to snowball and carry, but easier for others. Now I'm not saying all players who did good were carried, but some were, and those that weren't might be having problems because of the changing metas and the newer items. It's just easier to say that a system that allows for easier advancement is harming your rank.
All in all the new system provides players the chances to continue going up in skill, and proceeding forward. Rather then forcing them to play at lower and lower skill as they drop in ELO. As a player that climbed out of 800 ELO to 1300 ELO as support without a Duo partner, I know the sever problems that faced that system. I'm sure there are others who experienced similar issues as myself.
In closing I find the new system to be better then the old, but there is still room for improvement.
Flames of Terror