I've had 300 F/2.8L Canon AF lenses for several years, and have found that using a 1.4x extender is normally OK; using a 2x extender needs more care. Specifically, I've found that AF-MA needs careful optimisation, and that can take time. When optimised, I've had good results from both the non-IS, and the later IS versions.
The non-IS version of the EF 300 F/2.8 L was the first AF version, and it's what I bought as my first "big white" - as a well used copy, after using an EF 300 F4L (non-IS) for several years. I later bought the EF 300 F/2.8L IS.
Physically, the original lens is heavier and has some nice touches, omitted from the later IS version, such as the rotating strap mounts. It takes the older slot-in filter size, 49mm vs 52. Mine came with Canon's thickly padded case, but the padding wasn't sufficient to stop it being damaged by trolleying over rough ground - hence my need for a repair, and that proved difficult. A resourceful repairer was necessary as parts are no longer obtainable from Canon. (Lehmanns of Stoke-on-Trent, in the UK, sorted mine for me after another Canon approved repairer made matters worse.)