you don't have to be pushing near the limit for mix tyres to be an issue. mixed front and back is better than mixed across an axle.
its down to the fact that you'll be running different, tread patterns, compounds, sidewall stiffness among other things. you hit a piece of ice on the road, or a diesel spill. the tyre could react differently. one tyre travels over it fine, but then the other doesn't.
its your choice at the end of the day, IMHO the tyres are the only point of contact between the car and the road, and the only thing that keeps you attached to the tarmac. its not an area to skimp on, I'd rather run a good tyre on all 4 corners, than a mix of tyres or put cheap tyres on.
as said before its your choice, you can do what ever you like with the information, follow it ignore it, up to you.