A wider tire will slow the bikes ability to turn. (The #1 reason why the B6 is more nimble than the B12 in the twisties.)
A wider tire usually needs a wider rim. You CAN put a 170 on the B6's rear rim if you feel you just have to have a wider tire. A 180 will not fit the stock B6 rim. You also need to pay attention to clearance issues when trying to put a wider tire on that what was meant originally. You have to be sure the tire won't contact the chain, swingarm and the brake torque rod.
You aren't going to increase the payload of the bike with a wider tire, the stock tire is already cabable of carrying a heavier load than the bike is rated for.