Thinking of buying a Phoenix 500mm f8 w/ 2x teleconverter, any comments?

You are describing a mirror lens which will have poor quality to start and be even worse and perhaps even unusable with a 2x TC attached. There are some good consumer grade zoom lenses like the Canon 75-300 or Tokina 80-400 that would work very nicely for spring training, in fact you can get by with 200mm and some cropping. Here are some spring training photos I took a few years back with a 200mm lens and a 1.4 TC - from the stands (you can compare them to some of the other stuff there for minor league which I took with a media pass from the photo box).

http://www.olphoto.net/gallery/434507

I think what the previous post was trying to elaborate upon is that f8 is a pretty small minimum aperture, coupled with the fact that you are using a teleconverter you may have to overexpose by 1 stop which essentially reduces your minimum aperture to F16 which will then affectively increase your shutter speed thus blurring the motion unless you use a relatively fast ISO setting like 800,1600, 3200 iso. Look for a lens with a bigger minimum aperture like F4 and perhaps stick with 500mm and below. Also you may want to try KEH.com and look for a quality used lens, it is really a great place to shop. I have heard some bad things about Phoenix, but if you want to go for a non-mfg lens but something cheap check out Sigma or Tamron.

That lens would be a waste of your money. Make do until you can buy something better. Stay away from anything f8 and that far after market. Adorama & B&H also have used departments and with it being after Christmas, there might be alot of people selling because they got upgraded equipment for Christmas! I have picked up some really good used deals this time of year.

If you end up with 1000mm at F8 I doubt you would ever hold the damn thing still enough to take a realistic shot for sports.

I often shoot at 450mm F5.6 from the back of a boat and about 1/2 the shots are misses.

It is good but the diafrom will be shrinked to f16 and how will you shoot in low light? In the bright day time it is useful.

f8? are your sure. that is awfullly slow.and doulble that with the converter to f16 at 1000mm your object must be still... in bright sunlight and the lens with its own tripod.