To what extent does changing from 14 inch to 15 inch wheels effect the speedometer/od

It's all in the percents.
A change in the tire by 5%, will change the speedo by the same 5%. It is most accurate to measure the tire roll-out.

The same goes for any correction. If you are traveling at an indicated 60mph, and come to believe that your true roadspeed is 66mph; that would be a 10% error, and requires a 10 % change it pinion tooth count.