I think that 60x should be piece of cake for the EDIii.I boost my nikon ED50 to powers that are amazing,usinf my 8x binoculars,and at 1 km distance i am able to read a 40 cm sign with a telephone on it and discern most of the writing and numbers,size of the type must be around 1 inch,i guess...i am talking about 184x with a 50mm aperture..my scope is a bit better than average,because i tested many units and not all were able to achieve this .But a very nice regal 65 i owned was able of this as well,and i dont see the E D iii as being less capable.but of course,amd i agree here,buying an expensive eyepiece for a few times that you can find right conditions,or that you need this power,is not very economical...plus nikon eyepieces are not that swift to swap neither are they compact ,so two hands and a pocket are adviceable to proceed.....in that regard,the zolm,s ability to go back and forth in a sec is impossible to beat...shame on the eye relief...my suggestion would be to try one of those universal doublers,a route that i might explore in the future myself,with a 2x ,quality doubler you have access to reasonable magnification with your 30x ,probably with a very decent result too..
If you want to complement your eyepieces in the future,maybe the 24x ( 48x with your doubler)would fit a great niche....