1.4x teleconverter gives a 40% increase in magnification and 2x gives 100% increase in magnification. Main downside is that functionality of auto focus decreases with increasing strength of teleconverter. With 1.4x autofocus is noticeably slower and with 2x autofocus is virtually unuseable. I rarely use teleconverters these days and usually use wet close up lenses.
I am thinking about taking pictures like wild textures, and for that purpose I am looking for high magnification. Did you ever try to use teleconverter and wet diopters??
I see that You have Tokina 35 macro lens. Did you found that lens useful for macro?