If it had been an 800 or 900 upgrade, it would have been the same. It took a little longer because of the odd size, but we are talking 30+ year old bike with at best 50HP going to possibly 65. So the £10 yearly cost to me is very reasonable.
You could simply fit it and not tell anyone. However, if something really expensive happens then a simple count of cooling fins would identify the change. I doubt they would bother for written off bike and a bumper smash, if there was a fatality and multiple pile ups caused by me, then they might have a look.
If I were 30 yrs younger, the bike was a Hiabusa and I'd fitted a home brew turbo with nitrous, the premium might have been a little more.
I imagine there might be lots of undeclared mods, our bike inspection process is quite basic because it would be expensive and make no difference to accidents. A hot cam or performance exhaust is unlikely to affect the chances of SUV muppet pulling out on you.
Bike gets nicked, undeclared expensive mods? Still low risk to insurance company.
It's all down to risk and money. Which is the same as any government or corporate business model. Same in engineering.
I should charge a consultancy fee for this insight