I believe that buying Lens Turbo II is recently quite affordable and worth trying. I will suggest Metabones Ultra, mainly because of better optics and I would feel safer regarding possible reflections, but it is very expensive. You might invest in Turbo, see if you like results and if you experience increased flaring, you can always re-sell it with rather little loss and buy SpeedBooster Ultra.
Full Frame has its advantages, but so does APS-C. If the rumors are true and we soon see A6100 with 36 Mpx APS-C, IBIS and 4K video, I would certainly consider such a camera as a very serious contender to A7 series.
Just my 2c.