As Joe mentioned, 2x teleconverters are usually a fair compromise, even for the very best ones with very good lenses. When paired with more consumer-grade and travel lenses like the 18-270, it's not generally a good pairing except for emergencies. And almost certainly, you will lose autofocus capabilities (it's not just a Canon thing...virtually no camera will autofocus at over F8 - and a 2-stop teleconverter paired with an F6.3 lens will definitely be over the limit).
I regularly use an expensive matched teleconverter with a long prime lens - it's a 300mm F4, paired with a 1.4x TC. The results are excellent, but that's a matched TC designed for that lens, it's only magnifying 1.4x, and the lens is fast enough that with the 1 stop loss, I'm still shooting F5.6 which means I can still autofocus.
I do have a Tamron 2x teleconverter, however I lose autofocus on most of my lenses I pair it with - and the quality does take a noticeable hit in softness and contrast. Sometimes it's worth it for extreme situations, but honestly I don't use it much, and never on my more basic lenses like my 18-250mm. In fact, the primary use for the 2x TC is paired with my Tamron 200-500mm lens when I'm shooting moon photography, so I can shoot at 1500mm, more than enough to make up for the slight loss in image quality due to the ridiculous reach! But it's manual focus all the way, the stabilization system can't keep up with it, and you need a lot of light with the 2-stop loss.
