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I used opencv_createsamples to warp (with no background) the original images to have around 700 positive samples at the end

There is your problem, using that single step you created the most awfull and bad training samples ever. Better collect 50 natural samples than 5000 artificial ones. To know why I am stating this, read up on about every single response I made on this forum or read the recently released OpenCV 3 Blueprints book.