Short answer is : It's not so good... but it's for science !
Timestamps :
00:00 Intro
00:45 Quality with a 50mm Lens
01:16 The stetup and control Image
02:06 Some math you can skip
02:47 Sample with a doubler
03:15 What about Triplers or 3x converters
03:42 Let's combine the two...
04:28 What if we stack all of them for 2800mm ?
05:03 Outro
05:22 Some more ?