The battery's biggest responsibility is starting and yes as voltage goes down the windings are getting push because current required to start goes up. The battery does do 2 other things 1) it serves as a huge filter to resist changes in voltage so the ripple from the alternator is "flattened" and filtered by the capacitance the battery adds to the circuit. 2) It is a surge protector for the system in that its stored energy can give you the capability to deal with "short term" draws from it's stored energy in dealing with big short term loads like rad fans, wipers, lights, etc.
The thing is the energy needs to be put back in when it is taken out so if you are upgrading and know that you are putting more loads on the battery, upgrading the battery by itself is not enough. The alternator and wiring needs to be upgraded. You will shorten the life of system electrical components like the battery, winch, and starter. The same is true for them; as battery voltage goes down current goes up and things will start seeing more wear.
You need to upgrade the wiring as well. The most important thing in your setup is not increasing the battery it is the alternaotr. If the battery is bigger and your alternator is not big enough you will draw down the battery and be left hanging. The battery is essentially just a short term storage device. The lights at 30 amps are almost double what your rig uses normally. Upgrade the alternator and the wiring.