Quality weapons (White Giant)

Does a 300Q air to air missile have the same chance to hit interceptors as a 100Q air 2 air missile?

Is a 300Q interceptor harder to hit?

Same question for all other weapons.

Also do higher paid soldiers and officers use the weapons more effectively?

There is no known benefit to purchasing high quality weapons over low quality.

I do not believe that higher paid soldiers are more effective in combat, HOWEVER low salaries may lead to soldiers leaving their jobs in favor of higher-paying (if applicable) corporate jobs instead. Government salaries should generally be comparable to or maybe slightly higher than average corporate salaries to retain government workers. While deactivated weapons aren't a big deal, closed hospitals and schools are lost forever and you must order new ones to replace them.

I think the quality is just for the corporations to sell at a higher price and for money purposes

There is no difference... only price.