Having just driven on the motorway in the worst conditions I've had to withstand in the Zed, I'd definitely recommend spending the extra on tyres. It's given me enough confidence to keep driving, if I had bald/crap tyres I would have parked up and waiting for some sunshine.
Seen a few incidents leaving bits of bumper, gravel and all sorts on the motorway, definitely not a nice day to drive a sporty car. Guess it's a mixture of driving skill, driving sense and car tyres/brakes to match - like its been said above, if in doubt don't take it out.