What's the difference between the Pri fatboy Gastube over the Standard Gastube? do i need to get an pri fatboy gastube for my M4 14.5 bushmaster barrel?

The fat boy tube has a larger diameter over the barrel section of the rifle.  This increased tube volume softens the gas spike on unlock.

Do you need a fat boy tube, No.  The M-4 work just fine without it. Just make sure that you have the correct M-4 ejector spring in the bolt (black insert, not blue).

they're a better idea for really short barreled rifles. Such as anything below 11.5"
You should be fine!

I have two rifles with the Fat Boy, one is 11.5" and is a good edition.  The other is an M4.  I tell you this, don't bother unless you have a problem with extraction and can't solve it with just updating your extractor spring.  I don't think it is necessary on anything longer than 11.5" unless you are shooting a large caliber like the Beowulf or other some such, then it can reduce recoil.

Don't fix what ain't broke.