Ive ran stretched tyres for years and Ive not had a big problem, only problems ive had is always at tyre fitters that cant bead blast the bead of the tyre on to the barrel.
I've ran some daft stretch's and been trouble free, yet on my E60 BMW which has run flats from factory has cracked a wheel in 3 places, and thats using the correct tyre tyres size supplied by BMW.
Only thing Id say to be thoughtfull of is, that with running skinny tyre's/ stretched tyres is that you bringin the outer edges closes to the road so try and avoid driving like a loony, and I definatly dont reccomend driving it on a track on stretched tyres.