In reply to coachio:
One option is to use an SLR lens (e.g. Canon 400mm f/5.6 which can be had for pretty decent money second hand, and use a converter. Expensive converters keep auto-focus, but it's probably still going to be cheaper (and longer) than than the Sony tele-lenses (which currently only go up to 300mm and reviews were iffy).
I think Sigma also have an NEX-mount adaptor, so you could look at some of their long lenses which may be a good middle ground between Canikon lenses in terms of price.
It ultimately depends as to what appeals to you about mirrorless. I upgraded from a 7D to a A7R2 for both size and image quality.
I VERY rarely have a chance to go our shooting wildlife, so lack of long lenses wasn't an issue for me. When we went to Canada last year and I did need a long lens, I just hired one for the fortnight...
If you're looking at the A6300 for the size benefits, sticking on a long lens will not only render the small-form redundant, it also won't be a comfortable combo to hold.
If you're regularly shooting wildlife, I'd say the A6300 isn't for you. If it's an occasional thing, look at hiring lenses (and adaptors) as they're pretty good VFM unless you're hiring them every week...