On the Lightning the overwing pylons were a necessity because the MLG retracted outwards into the wing leaving almost no room for underwing pylons, drag was an issue however and the overwing tanks were hardly ever fitted to the F.6 fleet during their RAF careers, although they were refitted to the Warton test fleet (XP693,XR724,XS904,XR773,XS928)in support of Foxhunter radar trials with the Tornado.
-Dazza