I wouldn't ride that, or many other, stretch of the A30 for any reason, at any price.
That stretch is pretty straight and very quick - No-one's expecting to find cyclists there.
There are many alternative routes to get you from one end of Cornwall to the other, without chancing the Trunk road, but the End to End cyclists and walkers seem to have tunnel vision when route planning: I'd use CycleStreets to do the planning and Sustrans routes where possible.
The difference in time taken by using the trunk roads against a more scenic route is not huge when you're on a bike.
Just my 2-penn'orth.