Like with most good things, there's a drawback. The converter will also double the maximum aperture of the lens- i.e. the limit to how 'wide' it can get, and thus how much light it can take it- basically meaning less light; and its due to these compromises in image quality that the use of teleconverters has always been a bit of a marmite topic.
Still, with the help of a little sunlight at the right angle, I was pleasantly surprised at the performance of the combo on one of the resident Stonechats down by Pen Ponds. Definitely still 'presentable' images, give or take a few minor tweaks in brightness and contrast at home...
Whilst Stonechats are naturally approachable birds, it was great to get the extra reach on a species that wouldn't normally be as accommodating- a Green Woodpecker chilling in the afternoon sun.
The 120-300 lens has always worked well with my 1.4x teleconverter, and from looking at the shots from today, I'd say it performs much the same with the 2x. When it comes to photography, I don't like to dwell too much on the facts and figures of it all- in the end most of it does comes down to how the photographer uses the glass- and whilst the converter won't perform all the time, its still a great little piece of kit to have in the bag for that moment when you just need the extra reach... we've all had it.