Most hard hull speed boats come up on plane between 15 and 20 MPH. The sweet spot for my 18' Wellcraft to start to come up on plane is 18 MPH. Top speed is around 50 MPH with a 220 hp motor. The boat weighs in at around 3,200 lbs without people.
With an 8hp 2 stroke the fastest I've been able to achieve in that boat is 4.5 MPH. I had a 5 hp 4 stroke Honda that would only push the boat at 2.7 MPH if the wind wasn't blowing. It wouldn't make headway on a windy day. The difference in torque between the two was quite noticeable.
With the set up your talking about, (soft hull, lightly loaded) I would say your top speed will be around 10 MPH. Wind and chop will slow you down some. You won't be getting up on plane without a hard hull boat, so I don't see tracking as much of an issue, except on a windy day. I've never been on a lake with my raft and motor set up, so this is only a guess.
Good luck and have fun.......but keep the bottom in your boat. I can't imagine that mounting a motor in the middle of your boat would improve speed or tracking??? .