Highest theoretical power is 250x but it has to be a perfect night. konus does have very good optics but at f/4.3 it is a low power, wide field scope. So 500mm fl / 2mm eyepiece = 250x. Eyepieces of that high power have eye relief issues as well as blurry edges unless you want to pay $500 or more
Here is a good eyepiece which will give you 96x and 20mm eye relief. I own the 12.5mm version
http://agenaastro.com/agena-5-2mm-ed-eyepiece.html
BTW I rarely go over 150x with my big scope, the higher the power also equals seeing more atmosphere