As you are probably aware, insurance companies will use any excuse not to pay out on a claim.
Not so long ago, the financial ombudsman castigated the insurance industry for claiming that fitting a roof-rack to a vehicle represented a modification – and therefore denied claims.
If a vehicle had been modified with any of the items you list, I would expect the financial ombudsman to find in your favour (if they tried to use it as an excuse).
However if you disabled an alarm/immobiliser, they might very well have a case for denying a claim for theft.