That's a pretty high nozzle temp, even for PETG. I've never printed over 245 and my current filament seems to be happy with 240 for the first layer and 225 for subsequent layers. I would first try lowering the temp to see how low you can go and still get good layer adhesion. Another possible culprit is insufficient retraction distance and/or speed.
Also, do you have the Wipe feature enabled? I was having some stringing issues that even temperature and retraction changes weren't fixing, but they've all but stopped happening since I enabled Wipe. I left the wipe distance at the default (5mm).