"I'm getting some new wheels, thinking about 15x7 up front and 15x8 in the back. Will this cause understeer?"

With only that little of info, the most accurate simplistic answer would likely be, it will change the balance, whatever that currently is, and it will be TOWARDS understeer, which maybe is your objective?

Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.