I tell them everything, mostly they aren't interested, but I write to them with
every modification (no matter how small), plus a small explanation of why (so in the event of the DNA, it's washable for long trips
.... the MTCs replaced the failed OE cans as they were cheaper
) My modifications total as much as the bike is worth...
I have never had the bike on a Dyno, so I can honestly say I do't know of the performance improvement, but they allow something like 5% - I may be wrong on the figure, check with your company. In any case they worry about HP not Torque.....
It's never made any difference, but I see it from the other end, if I make a claim I want them to already know what extras are on the bike. So I have no need for this
with them!
The other aspect is a sharp traffic officer may well wonder if you have disclosed everything, technically if you haven't, then you are not insured and that's a ticket-able offence. He/she would have to be a proper t w @ to do this, but why risk it ?
Be honest - Only way.