The guard does reduce your fuel economy, especially if you mostly do highway driving. Removing it should help some (probably not a huge amount). Do you have other accessories stuck to the outside, like racks?
You can find out if 55mph will be a problem by trying it. If you don't find yourself settling in with other 55mph drivers then it's probably not safe to be going at too different a speed from the rest of the traffic. When I am in the mood to go slow on the highway, I generally keep it within 5mph of the speed limit.
I know having a third car is pushing it, but you could get a cheap used vehicle that had greater efficiency. Used car wouldn't need too much in the way of insurance and multiple car rates for liability are usually low. Or maybe a scooter or motorcycle, if your area and inclinations lend themselves to one.
: a psychological condition or a relationship in which a person is controlled or manipulated by another who is affected with a pathological condition (as an addiction to alcohol or heroin) ; broadly : dependence on the needs of or control by another
I do not recommend removing the cat. What will happen is that the computer will sense this, assume something is wrong with the system, turn on the check engine light, and run in "limp home" mode on a set of preset parameters. Your mileage will likely go down.
The tires are another thing. You may see a gain going to a tire with a less aggressive tread. I have highway truck tires on my K1500, and its performance off road and in snow & ice really hasn't suffered much. If the tires are due for replacement you may see a gain from tire replacement. Unless you have a really aggressive off road tread on your tires, or you have super swampers on it. Otherwise the expense of the new tires probably won't pay for themselves.