Porting and polishing is about increasing your airflow into the cylinder and out of the cylinder. We often change the shape of the intake pockets to clean up the swirl to give a more even distribution of the fuel air mix in the cylinder, reduce turbulence (to a degree, you need to keep some) and help keep the fuel atomized. Thing is, if you don't really know what you're doing, you can screw things up. Until the Vortec head for the small block Chevy, they all needed some porting work. When my buddy brought me a new set of Vortecs and asked me to do my stuff to them, I looked them over and told him there was nothing to do, Chevy finally got it right. A couple years later they scrapped the whole small block lol. True polishing one only does in the exhaust port, intakes I make no finer than 80 grit, and the sanding pattern matters (to a degree, only critical in full on racing applications where every .001 second counts). On an airplane engine that's only turning 2500rpm though, the only thing I would bother with is match porting the induction runner to the head runner including the gasket (it provides the template to match the two) to assure a clean transition. With a turbocharged engine, this all becomes extremely LESS critical as you have pressure on both sides of the system, and most all of this is about free flow and scavenging under vacuum. A clean match port is about all I would bother with and I would spend my money on a cam with a reduced split overlap to increase the turbo efficiency.
Balancing is all about reducing damaging vibration which besides tearing things up will hinder the ability of the engine to turn high RPM due to increased friction. Again, on 2500rpm engines, this isn't super critical within the engine, although I would weight balance the prison/rod assemblies to each other. More critical on aircraft engines is dynamically balancing the prop, this is low cost and high value.
GAMI injectors are about matching the fuel flows between the cylinders to match EGTs allowing you to run smoother and safer in the LOP ranges. You only need them if your stock injectors leave you with a pretty good temp spread between them. If you lean your first cyl 20 LOP and your last cylinder is at 50-75 ROP, you'll likely want a set if you want to run LOP otherwise you'll either be rough or put a cylinder into a risk of detonation.