Any lesion of the tympanic membrane only partially affects conduction. The tympanic/ossicle complex accounts for the majority of conductive hearing but the skull also has some conducting ability. As for after the procedure- the tympanic membrane will probably heal if the incision was small and done in the proper place for incisions but will probably be a little sclerotic reducing the conductive hearing through that ear, sclerosis of the tympanic membrane is very similar to what happens in the elderly and accounts for the hearing loss in the elderly.
To make a long story short- no they won't become deaf, just slightly harder of hearing (very slightly)