The 8hp Yamaha on the dink for my 34 will push the 34 (13,000lb) up to almost 5 knots. A 2hp Honda pushes it to just over 3 knots in calm water. There is a big jump in outboard weight between 6hp and 8hp. The 8hp is a detuned 9.9. The benefits of the 9.9 aren't noticed until the top end of the power curve and I don't run my outboard that hard. The 8 turns the inflatable into a speed demon.
Really, the only reason for the 8 is to have a backup for the diesel should it fail for any reason. Murphy says it will fail at the worst possible time in the worst possible place eg. in the middle of a set of rapids. It gives peace of mind knowing there is a backup and I'm not at the mercy of the tide gods. Also, the dink becomes quite exciting with the 8 attached to it.