As part of my old job (insurance broker) used to be to arrange marine insurance ( for individual boats & dealers) I'll give it my opinion .
We used to fund that most insurers in the UK are more concerned with max speed & where its being kept ( most claims are theft / malicious damage related , very few for real full on boating accidents - by number, not cost). We used to have to explain that to ask for more money when the max speed went over 50 mph ( yes they didn't understand knts ! ) was daft as depening on all sorts of things the same 2 boats with the same HP/ engine may or may not hit the same top end ( especially in RIBS where alot of 6m boats hover around that level WOT) And as you say for one offs its all very very subjective - there fore costly for insurers to think about.
As a result they take a broad brush approach - most are 100% standard and for anything not charge an extra £50-£100 a year ( more as a token - in claim / loss ratio situation its worthless)
Most of the proposal forms (of the few that exist) dont ask for max HP of the boat), for example the Nav & Gen wording exludes under the whole policy :
the cost of making good any defect in repair
or maintenance, resulting from work carried
out by any person employed by you.
• the cost of making good any fault or
damage arising from any fault or error in
design or construction.
• replacing, repairing or renewing a faulty
part, faulty design, faulty construction or
defective materials. - Can you class 250 hp on a 5m Rib faulty design ?
But under the personal accident section excludes :
wilful exposure to needless risk
So it would ( as I understand in the UK ) come down to the insurer ( the burden of proof is technically on them, but in real terms it will be down to you) or you proving you haven't had cover excluded under these type of wordings.
In these cases a good broker is worth thier weight in gold as they can get very technical on these wordings on your behalf (although remember they dont have to - hence get a good one! Ask them exaclty how they would handle claims when you buy the policy) .
So an extra 5kg / 10 hp on the transome probably wont make a differance ( you may to prove it !) , but you could argue that having en extra 150hp on the back (over the max recommended -its only recommended after all ! ) doest make a bugger of differance to it getting nicked as long as you told them up front & they charged you correctly.
You can get into long arguments over the the 'disclosure of facts' under the idea of 'upmost good faith' if you are not 100% honest , but generally you'll get 90% of things covered under std policies & the extra 10% you'll get covered but you may well have to pay that bit more for it.
If you are really bored have a read of this
and see if you can find any exclusion for extra power / bigger engine . Its very hard for an insurer to write this in otherwise they could insist you have x number of red flares on the basis the boat may have been salvagable if the RNLI found you sooner etc - just unworkable & too costly for an insurer to administer. A lot of polcies ask that there is a 'competent person' onboard the boat , then will state thisis considered someone with at least x years experience - again nice and vague so they can argue with you if you are a dick & taking the piss!
Legally you will find most harbours/ marinas insist ( on paper at least ) that boats using them carry liability cover to a certain amount - but again virtually un-enforcable for visiting boats.
Hope this helps & yes I am this bored in the office today !!