We'll try to give it a shot without writing a whole chapter in a book. Caster is a huge element in weight jacking as the kart corners and controls the timing of weight jacking - which corner unloads/loads when. Rule of thumb: if you are running on a short track with tight turns you need to run lots of caster to get a better run off the corner than with lower caster. On a sweeping high banked tracks you need to run less caster to get the same effect.
Increasing caster split (the difference between LF and RF caster, 4 degrees in your case) results in a larger percentage weight loss/gain (depending on which front wheel you are looking at), basically controlling the amount of weight transfer with a given angle of steering wheel input. More caster split will tend to free up the kart because less weight is transferred to the right side tires by weight jacking.
A 1/2" kingpin is a good deal stronger than a 3/8" kingpin, so when you hit something, it will be less likely to bend - or, conversely, more likely to cause something else to bend in a collision, due to its extra strength. Other than collision performance, I can't think of any other advantage of 1/2" over 3/8". If there are other advantages, others will chime in.