Lenses with TC's are just like any other lenses- while they might degrade performance some, at others, they might actually improve performance. Under certain circumstances, stopping down the combination- helps. 1.4x TC's tend to focus faster. 2.0x gives you the range you need sometimes. I guess I am fortunate- both the Sigma and the Canon 70-200's take TC's fine.
Now, about possibly working better than expected (improving performance... Any lens and body combination- is either spot on, off one way, or off another- and depending upon how much- IF- determines how soft or "on" the lens is. MFA can help, if you can get it right- and if the body supports it. Now if you throw a TC in there, it's also got an effect on that overall "balance" and it might put your combination "right" or "spot on" where you could notice a slightly off- bare. So, technically, a TC can actually improve things... I carry two TC's with the 70-200 rather than carry a 70-200 and a 50-500. Sometimes, I take out just the 50-500, but it's a bit hefty, especially after an hour or two. Mostly I use a 1.4x TC with the 70-200, if I can- maintaining most of my AF speed.
EOS 5D III, EOS 7D,EOS Rebel T4i, Canon 70-200 f/2.8 IS II, Canon 24-105L, Canon 18-135 IS STM, 1.4x TC III, 2.0x TC III, Σ 50mm f/1.4, Σ 17-50 OS, Σ 70-200 OS, Σ 50-500 OS, Σ 1.4x TC, Σ 2.0x TC, 580EXII(3), Canon SX-40, Canon S100
Fond memories: Rebel T1i, Canon 18-55 IS, Canon 55-250 IS, 18-135 IS (Given to a good home)...