I think it is a cracking lens, use mine with both 1.4 a 2X TC's sadly I haven't had much time to go out shooting birds over the last couple of years so wondering about selling mine so someone else can enjoy it.
Here's my take on the lens....I owned a Sport version for about 2 years of extensive use. The lens itself is great and holds up well quality wise. It's definitely on the heavy side and even though I handheld the lens all the time it's tiresome and takes a toll by the end of the day. I had no hesitations using the lens wide open at 300mm, it's very sharp.
TCs...they are a different story I'd say. The 1.4x is still solid even wide open, but would prefer a stop down to f4.5 at least, especially at longer distances. The 2x I would say is border line usable. Up close distances are fine, but anything more than say 20m away, you lose fine details...I'm talking from a wildlife/birding perspective here.
My conclusion would be this: If you plan to use the lens by itself or maybe sometimes with a 1.4x TC, then go ahead, it's a great lens worth the money, but if your main use would be with a 2x TC, you're better off buying one of the 150-600 zooms.
You'll need to phone Sigma UK direct and be quick as they've a limited supply. It'll cost around £25-30.
They are, however, about as useful as a chocolate teapot.
Whomever designed a thin, lightweight, plastic lens hood to go on a two ton megazoom needs their head examining. Or at least the sack.
THe reason why your lens didn't come with one is because the original almost certainly broke.