Hello and welcome to the forum.
You seem to have gathered some very useful data on the two outboard sizes and different props, far better than guessing or wondering. Where do you use your boat? River, lakes, sea?
Like you, as an older guy myself, weight is crucial so I've looked into and/or experienced many options in the SIB and outboard sizes you mention.
My air floor SIB weighs 43kg and we use two outboards. A 15hp 2-stroke Yamaha at 37kg and a Tohatsu 9.8 2-stroke like you mention. The difference in weight between the Tohatsu and your Yamaha 8hp is not worth worrying about, in fact if you put them both on scales they may weigh the same. I've found makers declared weights are often a kg or so different to their actual weight so don't worry about 26 vs 27kg.
We usually refer to speed in kts but converting to km/h we get 28km/h with our Tohatsu 9.8 carrying 12l fuel, kit, small dog and two people. We previously used a Mariner 9.9hp on the same boat and the speed was near identical.
The standard 8.5" pitch on the Tohatsu 9.8 wasn't quite right and we've found it better with a 7.5" pitch. Not so much the top speed but the responsiveness getting on the plane.
However while I can still lift it I'll mostly use the Yamaha 15hp 2-stroke because at 37kg it is manageable and gives a more relaxed cruise plus much better response to work through a lively sea.
I don't think changing the SIB to gain just a few kg is worthwhile. If your air floor comes out the best weight saving is to pack and carry the SIB with it out. Doing that brings the main boat pack weight down to 35kg for us.
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