Dave - I don't know Simcoe at all (other than the one trip out with Brian), but on lake O I have only caught a laker on a dipsey a couple times. Depth - 80 & 95 ft; wire set up 180 / 220 foot back, set on #3, black and white spin doctor, white fly, long lead between SD & fly (36 and 42 inch), and doing about 2.5 mph on the GPS - in June. I will run the dipsey back till it bangs bottom and make a note some days just to get a feel for how deep it is running. Picking up a copy of Precision Trolling will also give some good insight- Just remember once you start getting that far back and out, every element gets magnified - especially the sub surface currents.
Keep running them, they will produce. This year for me dipseys have under produced every other presentation - but usually they will boat the biggest fish.
Be happy to take you out on the water during laker season next year if you're interested.