I think the effects on any ride would be negligible, honestly.
And regarding Gringotts and Diagon Alley, I actually fear this will increase the congestion in those areas slightly. Considering it's a virtual queue, if someone has- lets use 30 minutes as an example- left on their time, they might decide to just wonder around Diagon Alley while they wait for their time to come up. They certainly aren't going to go get in line at Despicable me or MIB.
DI being so close to F&F might have adverse effects. Again, it will likely be negligible- but if I see it doing anything- I see it increasing congestion instead of relieving it.
I've said it once and I'll say it again- Doing a virtual queue on this ride was stupid and short-sighted. And if demand for the ride (i.e.- attendance growth) outweighs what the pre-show queues will hold, then it's going to actually make lines worse for other rides.