EDIT: I saw 300mm f4, 1.4 TC, less than $550 in the same post and assumed you are talking about an old Canon manual focus FD L lens, not a new one. If that is not what you meant disregard this post.
I used to own a 300mm f4 L and later replaced it with a 300mm f2.8. Both were used with a 1.4 TC and a Sony NEX-5N and later with a m43 body. I have no experience with the Tamron.
With both lenses there was an increase in sharpness 1 stop down from wide open. They were certainly not bad wide open and had very good contrast but I would hesitate to call them tack sharp unless stopped down a little. They might have been on a full frame body but the combination of a m43 sized sensor and 1.4 TC is like adding a 2.8TC which is asking a lot from a 30+ year old lens design, even if it was state of the art 30 years ago.
I mostly bought the lenses to play around with and after a couple of years of taking pictures of birds and squirrels in my back yard sold them. I was not disappointed in their performance but just got bored with them.
I would still recommend them over the Tamron due to resale value. I had a lot of fun with them while I had them and after they went back to ebay my total cost was very low. You can use them as a stopgap until you can afford a better native lens without losing a lot of money.
Is the Tamron native for your camera? If you can get fast autofocus it would be a big plus. Focus peaking and taking bursts while moving the focus point works ok on static subjects but can be a real challenge with moving ones.
These are the full image and 2 crops with the 300 f4 L with a 1.4 TC and a Panasonic GX7. I wish I could tell you what aperture I was using for sure but did not write it down. It is what I remember the lens and TC looking like wide open but it might be a stop down.
I also posted some test photos I took with the lens and TC at various apertures at
https://www.dpreview.com/galleries/6408103458/albums/boringtctestshots using a Sony NEX-5N. The shots of the leaf are all 100% crops with the 300mm f4. The orange vest was the 2.8 lens.
BTW, these were all taken using an electronic or electronic first curtain shutter with the lens on a Gitzo 1325 tripod and Arca-Swiss head on a concrete floor so vibration was not an issue. Used tripods are a great deal.
Tack sharp, 420mm and less than $550 is a two out of three sort of thing. Don't expect $5000 results from a $500 lens. But you will get much better image quality than a xx-300mm consumer grade zoom and cropping.
