I wouldn't if its a road car. If you have a bad accident and the car is taken away or you are unable to swap it back then the insurance company look at it there is a huge possibility they will not pay because the car by law should have a CAT and you have taken it off there for your insurance is invalid and they will not pay.
Now don't forget its not just the money for the car, your insurance covers any damage to building or other people, you could end up paying out for years to come just for a few BHP.
And a 100cell to decat is not much of a gain in power, std to 100 cell is still a noticeable gain.