One thing you did not mention was the lakes you fish. Speed is more important on larger lakes like Texoma and Tawakoni. On Tawakoni i can run 15 - 20 miles wheras on Lavon 1/2 that.

I have a FishMaster 196 with a 125 Merc. It tops out at 36 mph. I'm not sure what prop is on it but it comes out of the hole quickly and I typically cruise in and around 30 mph. I have a Lowrance HDS with sidescan and LSS transducer. Your ability to pick up return at speed will have more to do with the location of your transducer. I cannot get a decent return over around 8 mph, so I'll run from structure to structure then typically troll at 3 to 5 mph looking for what I want to see. Speed was not a factor when I was boat shopping.

I don't think any sonar is very accurate at more than 5 mph. But the faster she will go the more areas of a large lake that could be checked in a day. If you are going fast a large school would look like a drop in the bucket and you would probably never stop to fish it.

You'd be surprised