I have a 18" barrel and a 1:8 twist rate, I would like to use a 55gr. bullet. I believe that this rate is a little fast for this weight bullet. Would I notice that much difference?? Would it really matter?
I'm assuming that you're loading for a .223 rifle, yes ? The 1:8 twist rate will work very well for a 55 gr. bullet. It will be enough twist to stabilize a 62 gr. and probably a 70 gr. bullet as well. You can't really have a twist rate that's too fast to stabilize the bullet, but if you load bullets that are very fragile ( say, bullets suited for a .22 HORNET ), or bullets of lower quality that may not be weight-concentric, the faster twist rate will hasten their off-axis spin, and therefore you will notice tumbling and keyholing sooner. If you are loading a quality bullet, you will have no problems.
To start with you can not over stabilize a bullet. That is, there is a minimum twist rate and anything faster will not affect stability adversely. But there is another problem with a fast twist barrel.
The twist rate spins the bullet (obviously) but the bullets are only made so well and have a limit on how many RPM's they can stand before coming apart. It is about how the bullets are made. It is also about how fast they go through the barrel (the faster, the higher the RPM's). Hunting bullets typically have a hard time with the high RPM's whereas varmint bullets (and others) typically can stand a little extra rev's.
The results, if the bullet even reaches the target before exploding, can be poor accuracy. The way to be sure is to try them. I'd also try some at a couple of hundred FPS slower as well, to see if that helps accuracy. It is better to use heavier bullets with that fast of a twist but if all you have is the 55 grainers then they deserve a try. Just be prepared that they might not even get to the target or will have terrible accuracy. I went through this with my AR with a Lothar Wathers 1 in 8 twist barrel, so I have been there, done that. I was using a hunting 52 gr bullet that literally exploded on its way to the target. Lower velocities helped but the accuracy was very poor. My final choice was the Hornady 75 gr BTHP Match bullets with Vitahvouri N140 which my AR Varmint really likes.