My understanding of it is that if you don't declare mods then you should still be covered for standard parts only which is generally fine because standard parts usually cost more than aftermarket anyway.
However if you take the ****, as in the bike is almost not standard any more then you might have a problem with a claim, for that reason both my Ducatis are insured as 'custom' with all mods declared.
Initially it didn't cost much more than a standard policy but, as is the norm for insurance companies, they gradually ramp it up each year until it gets expensive in the hope that you can't be bothered to change - and this is where they get you because whilst I'm quite happy to change insurance for the better deal (do it most years for my car) it is more complex with a 'custom' policy as you can't do it online.
Instead you have to spend hours giving the same info to every insurance company just to find out that most want to charge you more, or won't cover you anyway, so you lose the will to keep trying (or to live) and stick with them ...
