My wife used to work for an insurance broker, this is her advice.
If you try to claim £750 for the Akrapovic that you didn't tell them about, not only will they not pay it, but they will say your policy was overvalued by the cost of the standard exhaust that wasn't fitted, deduct that from the insured value and pay only the pro rata amount. If the bike was stolen, they may deem that the after market exhaust increased the bikes appeal to thieves, and that you need to share some of the burden, and again, reduce the pay out pro rata. If this sounds complicated, just remember that insurance companies are just bookies. You are asking for odds based on the information you supply, if you don't tell them something that changes the odds, don't expect a bookie to do you any favors.
The rule of thumb is if your mod changes the value, the replacement cost, or the risk (of crash, theft etc) and you don't tell them before the claim, then don't mention them on the claim. If they find out some other way, expect a problem.
The most common mistake on household policies is under insuring. If you pay a premium for £10k cover, and your property is worth £20k, then you have only covered half the risk. Even if the claim is less then £10k, you will only get half the payout!