www.apexleisure.co.uk are the UK dealers for VHM and have been using them for ages on the NSR's they've done. Depending on the model and what gas you are using they add anywhere from 5-8bhp. The main advantage is that you never need to buy another head again if you have a seizure and the piston/piston rings hit the head. All you do is buy a new insert, much cheaper in the long run although more expensive to buy in the first place.