I'm not sure there's too many Great Persons by themselves, but there may be a problem when you combine it with other aspects. For example, the Golden Age extension wonders, Mausoleum of Mausolus and Cheomseongdae.
I generaly dislike using GPs for bulbing techs as you mention, it always seems like a waste to me. So, when you combine a civ that builds both of those wonders (more on that later) with all the free GPs you can get from the techs and add in my playstyle, you can get some insanely long golden ages going. In my current game, I had a GA going for well over 200 turns both from events, spending GPs and wonders, it was insane and broke the game. I'm going to have to purposely not build those wonders anymore as they synergize too well with my play style, leading me to game where I'm completely stomping on the AI if I build them and manage to get some/many/all of the free GPs from techs which can happen if you're the tech leader and already starting to pull away or are beelining for specific techs just to get the free GPs.
So to your question, by themselves I don't think that the many free GPs are a serious problem although as you say there may be a few too many of them right now, but adding in other factors can make for some real issues if you hit things just right (or wrong depending on your viewpoint). If possible, they should be spread out a bit more or reduced in number if that's the easier solution.
Now for the "later" - those two wonders I mentioned need to be either toned down a LOT or widely separated so it becomes very difficult to nab them both and not have both open up at Calendar. I went against my better judgement from past experience and built them both in my latest game and I regret it now as it broke the game open for me - the AI was just left in the dust as I was in the loooong GAs I mentioned and simply powered past them, founding city after city and taking out barb cities and weak newly settled civs as I came to dominate my continent. They are too good as it stands now, and if you get them both look out.
The only reason the game was even remotely close at that point was some pretty serious tech diffusion help the AI was getting (which seems to be working nicely btw), without that I would have been a full era ahead or more, with double or even triple the number of cities as any other AI on the map and then it really would have been a joke at that point.