TC-17 vs TC-20 on 70-200 VR ?

I have the 70-200 VR and TC-17EII. I'm thinking of picking up a TC-20E for more reach when the light is good. Would like to get a longer lens but can't afford one right now...

My question is - would there be an improvement in IQ at 400mm with TC-20E vs cropping the 340mm shot with TC-17E ? Or does the TC-20 degrade IQ so much on a 10 MP body that I get the same resolution in the cropped shot ?

Has anyone done this comparison ?

Thanks so much

Mike