Roast beef in slow cooker for 7 hours on low then on high for another hour?
There should be no problem cooking for an extra hour or two on 'high'. The rule of thumb which I have learned is one hour on "high" = two hours on "low". Generally, you should always start with an hour on "high" in order to get the food out of the danger area for bacterial growth as soon as possible, after which you can reduce the heat to "low".