reducing exhaust line?

I plan on using either a 10 inch or 12 inch fan to exhaust heat and humidity out of room and was wondering if I use a 8 inch reducer at the end of the line with it affect the cfm output of the fan much?

ANY reduction in size creates back pressure and/or increased velocity. One will reduce the amount of discharge and the other will increase the sound of air movement.

It will decrease the cfm, however not enough to make a major difference. It may also increase noise levels.

I'm thinking about doing the same thing.
Yes, air velocity will increase, but a fan of the smaller size will need higher rpm to reach the same velocity. And hence produce even more noise.

So lets look at a few numbers, area of a circle:

8 inch = 50.24 sqin

10 inch = 78.5 sqin

12 inch = 113.04 sqin

Going from 10 - 12 inch gives a 30% increase in area. 8 - 10 inch is a 36% increase in area. 8 - 12 is a 55% increase in area. Thats a mighty big minus IMO.

Your math checks out.
It's definitely worse than havin a bigger hole in the wall.
In absence of said hole, it should be an okay moove.