I did not mean to suggest that the 70-300 is crap. It is quite good at focal lengths shorter than about 275. It also decent at 300mm if you stop down to f/8. At f/5.6 and 300mm the lens is soft, even in the center, and lacks contrast that it clearly does have at shorter focal lengths. If one mounts this lens to a 1.4 TC and shoots it at 300mm and f/5.6 (f/8 w/TC) it will perform poorly. It can be improved a bit by stopping down to f/8 (f/11 w/TC). That is going to push shutter speed down, or require high ISO, both of which will negatively impact the resulting image.
I agree with you that some people have proclaimed this lens a hidden L. One of two things fall out of that observation. They are either delusional, or have never shot an L lens. I used the 70-300 on an XSi, T1i, 60D, 5D and an Elan 7 film body. It performed equally, as stated above, on all of them. I do not have extensive experience with L-lenses. I currently own a 100-400L, and in the past have owned a 135L and a 200 2.8L. All three of the L-lenses I have experience with are hands-down superior to the 70-300 in every conceivable way, except price. The 70-300 is a good consumer-grade lens, that falls apart at 300mm, especially wide open. To my knowledge it has never been referred to as a hidden L by anyone who has actually used an L-lens.
I just sold a 55-250 that was better at 250 f/5.6 than the 70-300 was at 300 f/8. The OP would be better served by spending $100 on a used 55-250 and cropping than by adding a 1.4x TC to his 70-300.