I did little experiment to figure out what is better to carry all the
time with me big and heavy but zoom Sigma 80-400 or light and sharp
Canon 200 2.8L with 2xTC.

Setup:

• Tripod
• Canon 40D
• Canon 200 2.8L
• Sigma 2xTC EX DG APO
• Sigma 80-400
  4-5.6 EX OS
• ceramic toy house

Conditions:

• Fluorescent light
• ISO 100
• RAW
• ACR4.2
• all preprocessing like noise cut or sharpening turned off
• 10sec timer release
• OS for Sigma is off

Enviropment:

Results

4 meters distance

2.5 meters distance

1.6 meters distance

Conclusion:

It is possible to use 200 2.8L with 2xTC it can produce better IQ in
term of sharpness, still focuses pretty fast. This setup is pretty
light, probably 2.5 time lighter then Sigma lense alone. Negative thing
spotted noticanle CA. Sigma is really grea, no CA at all.

Additional observation

I noticed that images produces with 200+2xTC are slightly magnified!? What does it mean?