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Not knowing the xA very well, there's lots of things you can do that'll help, and some that'll make things worse. Warm air intakes (piping to the exhaust manifold to bring in hot air instead of cold air) are not universally good. Some engines just don't like them. My GP doesn't, but my truck does.
Disconnecting the thermostat isn't a good idea, as many things rely upon it. Forcing your transmission (assuming auto?) to stay low while cold is hard on the fluids and could lead to premature failure. Not sure how to fix this, other than make a tranny block heater to warm it up faster. I have this issue, and at -20 it shifts kinda hard for my liking, even when mostly warm.
grill block good. do a nice one and it'll increase the aero of your xA as well as warm up the engine. There is no universal % gain, it varies vehicle to vehicle.
Ignore the electrolysis. Some say it works, some say it doesn't. Still waiting on empirical evidence (ie a closed loop track test) showing the differences. Mixed highway/traffic light driving is too varied for real testing.
Lowering the car decreases your frontal area. Less air to push, less HP required to move it. It's a good idea IF you think you can save in fuel costs from the lowering springs. $150 in gas get's me a pretty long distance.
There's a start to answering your questions. Others will have opinions and facts to help as well.
Welcome to the forum, hope the knowledge gained here helps you push the xA to above 45mpg. That car has potential.
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