How do current Mandarin speakers usually pronounce 阿Ｑ正传?
Generally speaking, in modern practice, Chinese speakers pronouce mixed English letters as is. for example:


  5A级景区


is pronouced as "wu3 ei1 ji2 jing3 qu1"

阿Q正传 is somehow different, because the author had explicitely mentioned in this novelette that Q should be pronounced as 桂 or 贵. (Note that back in the era of this novelette Chinese people were just starting to get familiar with western cultures so there was no established rules as how to incorporate English letter pronouciations in Chinese context.)

What I have experienced, when famous theatre companies present the play of 阿Q正传, they were always faithful to the original novelette and pronouced "gui4" in the play. However in daily speech, people sometimes just follow common practice and forget that this should be a special case.