That's just your tire guy's laziness talking.
I agree the 295's all around should be pretty good, especially with the stock suspension setup. That's why I won't be sad if the 315's won't work, but I think they will. And I'd rather start with as much tire as possible and tune my way into the balance with alignment, bar, and shocks than do the same with the 295's all around and have to start over if I find out I need more tire. I have a feeling the 315's with a little more camber will be helpful in slaloms where this car tends to wag its tail a little too much.