Bigger rotors dissipate heat faster, so they work better under high loads. They also generate more leverage, so they have more stopping power. This additional power is more noticeable on the front brake than the rear. Upgrading from a 160mm rear rotor to a 180mm rear rotor will not have the same effect as upgrading to a 180mm rotor up front. They have the same theoretical impact, but in practice, the bigger rotor up front is a more noticeable change.
I.S. adapters have two advantages over the other adapters. They don’t require you to thread the brake right into a frame or fork, and you can unbolt the adapter and take the brake off without messing with its adjustment. This is only really handy for traveling, but I suppose it’s worth noting. The downside to I.S. adapters is that they are inelegant, and the bolts that mount the adapter install from the side. This means that all of the stress generated by braking force is transferred to two M6 Bolts, which are threaded. Threaded anything is basically a stress riser waiting to happen. In practical purposes, these bolts don’t snap. But why design a system with a built in stress riser.