If you lift the cover you will see the counter works off the drum. Having a smaller diameter of line loaded on the drum would mean less dept than indicated, having a larger diameter drum loaded with line would mean more dept. Smaller drum diameter of line probably would only be significant when fishing deep. This could be corrected by increasing the diameter by loading more thinner downrigger line on the drum or placing backing on the drum. I am not sure if this is a significant enough issue to worry about. It would be interesting to hear from Scotty as to what diameter of drum results in the most accurate counter reading.
I have used stainless, 200 lb Tuff XP and 150 lb PowerPro. The Tuff line worked fine, however, the PowerPro definately has less resistance and I have had no breakage issues. My catch rate is not any less since switching from black box/stainless to braided. I prefer braided, however, have lost one ball to the braid being cut by my trim tab when turing hard with the tabs down. Otherwise, no other breakage in two years.