Thank you M, v interesting. I guess you may be a Naval Architect, or similar?
"I think we can assume the displacement of the boat with new engine will not differ much from it's current displacement."
I was really thinking about variations in crew weights, personal effects etc. singlehanded versus family cruising, for example.
Here the associated changes in displacement result in a relatively large changes relative to lightship weight. (You could easily load 500kg, imo?). To perhaps risk a "dance on the head of a pin", this will affect waterline length and resistance, to some degree.
Of course, the engine forms part of the light ship weight, strictly speaking.
OP.
Simply put: fully loaded, against wind and/or tide, you'll be pleased you have more power in reserve (for several reasons, some of which M has highlighted)...question is, imho, is the additional cost "reasonable", given (as M rightly points out) associated increase in engine weight is in itself negligible.
I am a Marine Engineer NOT a Naval Architect
therefore not expert!
...and then there are medium term power output & other "losses" due to aging of hull and machinery...that's more my area, unfortunately!
At the time, my Honda 5 was about 35% cheaper than a 6 and with immediate availability not a 3 month wait. I had no engine at the time...
Honda UK site today shows an 8 at £2445, and a 10 at £2515.
A 10 for me then please! However, both are out of stock.
https://store.honda.co.uk/marine.listBoth with v similar spec, and both at 42kg.