ISO400? Not too much difference outside in the sun I would suggest. This stuff is from a Wai Wai reloaded with ISO400 XP2, and looks just fine:
Wai Wai with XP2 Results.
While I would not suggest modern films have that much latitude not to make any difference, they come pretty close. Your disposable being similar in lens speed to a Wai Wai that is...
Also, while this tutorial notes the dangers of the flash circuitry, this should not be underestimated! I have added some extra words here as it jogged my memory of previous nasty experiences:
Filmwasters Wai Wai Reloading Review.
Look forward to your results! Skj.