No it doesn't affect. You got a tamron sp90, say it goes down to 20cm, you get "180mm" and 20cm focusing.
But nothing is free. 2X TC degrades optical performance. You get a larger image but optical details/sharpness/actuance degrades. But for sharp glass like this at optimum aperture there is still enough optical resolution to go around. And now since you get a larger picture the "noise goes down", luckily. Eg, last time your subject is say 1000 pix tall now it's 2000....you shoot at iso400 f11 (for macros, so stop down) it is going to be pretty noiseless if you resize for web to 800 pixels. So it might be a good thing after all esp for good glass.
Say for the tamron sp90, without 2X TC you stop down to f5.6 gets you real decent sharpness (2.8 is softer), now you might need to try f11 with the TC. Coz with 2X TC you optically crop it f5.6 actually becomes f11 (2 stops). If you shoot at f5.6 with 2X it's like f2.8 without the TC.
1.4X TC slows you down by 1 stop.
Of course if you have say a sigma 70-300 super macro II, don't try a sigma 2X. f5.6 wide open the sigma is soft but still manageable, f11 gets you good performance. Then with the TC you need to stop it down to like f18. Wow. ...
Do note that autofocus would most probably fail if you stack a 2x on your 28-300, cos the effective aperture is too small for autofocus to work properly, and the resultant viewfinder image might be too dark for you to manual focus properly. You can still try though.