Do people who handle blindfolded chess play at reasonably the same strength given enough time to think? If time is not a problem, can someone who is playing blindfolded chess play with (at least) the same strength than normal chess?

I have heard some people that mastered blindfolded can even play multiple games at the same time, so this makes me think they can actually get to play at their normal strength. Are there any studies on this?
In my experience and opinion


Having more time is not really an advantage when playing blindfold
You have a clearly higher risk of blundering when playing blindfold


Let me try to expand on this.


Playing blindfold in a single game for more than an hour for the entire game is a huge effort of concentration. Playing blindfold for two or more hours on a single game might be too much to handle, unless you have some special method or extreme stamina. So having more time isn't great from the stamina point of view.
There is a clear risk of losing track of something (e.g. a pawn) on the board or forgetting to register one of the moves and later pay for this mistake. The risk for blundering is clearly higher when you don't see the board and the pieces.


My conclusion is that you will definitely play below your full strength. How much below, depends on how well trained you are in playing blindfold chess. If you play on two or more boards simultaneously, I predict that your strength drops even more. My guess is that you can come up to 80% of your full strength when playing blindfold (after a lot of training and practice). Even then, your risk of blundering will be higher!