I have never had a problem by using cloud to race fully upgraded in WTTT, then restoring once i set times.. I would however never try to also 'cloud buy a car' i dont have as this i can see Monkeys frowning on this.. As i have all cars, i don't face these issues..
I have however received a warning one time during a team event, when i fully upgraded on my tablet to experiment in a speed event i think, but i was also reading the team wall on my backup iphone.. The rr3 message, on the iphone, read something like "conflict between devices" and one time trial was reset.. It also mentioned about this being a "warning"..
So, in team events, or even if doing WTTT, do not use two devices if using cloud assists.. Rule number one..
In teams, do not restore back till event closes, i would recommend, but TK reckons its ok to restore during the event still BUT not racing with that car again!