Shucker, I have the furuno 858, which is the same as the 620, but in the larger version. If you can fit the larger one, go for it. The display is out of this world.
I have the transom mount transducer. Didn't want to do the hole in the bottom thing for the thru hull. This works fine for me. I have had no problems with it picking up readings at any speeds. It is 600 Watts as they don't make the 1k watt transducer in the transom mount. And just an FYI, the 620 does not take the 1k watt thru hull, if that was the reason you were getting the thru hull. The money you will save on the transom mount transducer will cover the extra cost for the larger unit. Just a thought.