It's obvious when you think about it.
Put a 1.5x tc on a lens and you take it for granted that you are going to lose slightly more than a stop. Put extension tubes on to magnify another 1.5x - same. Crop factor is no different - there is no free lunch. If you think your classic 70-200 f2.8 is giving you 105-300mm and f2.8, you too have been successfully ...... what word should I use here, brainwashed? gulled? confused? led astray... you decide! by camera and lens marketings smoke and mirrors. You can't multiply the focal length by the crop factor and NOT multiply the aperture by the crop factor . As
"it breaks the math".
Your 70-200 f2.8 equates to 105-300
f4.2!!!
Sorry.