7" props have a disk area that is almost twice as large as 5" props, so the motors will be heavily overloaded. (7 squared is 49, 5 squared is 25, the other terms in the equation for the area of a circle cancel out)
Assuming you use 5x4.5 3-blade props on a 5" quad, you could reduce the load 33% by switching to 2-blade props. You can reduce the load further with a lower pitch prop. 7x3 is about as low as you can find, and will give you another 33% reduction, so now the motors are only running at 133%.
I think that's worth a try, but if the motors get hot you may need to step down to 6" props. 6x4 props may work better than 7x3, giving you more speed.
Reducing throttle to 70% will reduce the maximum voltage, similar to switching to 3s batteries. However torque and heating are related to current. Reducing voltage will limit the maximum power and thus the maximum current, but won't make any difference at part throttle.