Hmm, where does this information originate from? I have one installed, and I don't even realise it's there. It tracks your speed, so say if you're tracked at being on a small street or area doing 50mph, it might bring your score down. But no way can it tell that you're going 1mph over the speed limit, it would have to have a program telling it what roads have what speed limits and I'm pretty sure that doesn't happen. I've driven over the speed limit for the road I'm on and nothing has been said, so long as I don't go over 100mph, they don't bring your score down just for doing 41mph in a 40mph, that's ridiculous.
As for things like sharp braking and steering, isolated incidents i.e. a month or few weeks apart or more, don't count towards your score because that could be unforseen circumstance such as someone in front braking harshly/someone stepping out/a car coming out without looking. Your score only gets marked against when you brake harshly all the time, because this indicates that you tailgate or leave it too late to stop at lights, thereby increasing your risk of whacking someone's behind.
Accelerating harshly is an environmental thing, and you waste petrol when you rev past 2500rpm at low speeds (below 50mph). My score at the minute after driving with the box for a month is good to excellent, and my only downfall is that I drive at night (I work nights, so what can ya do?) but this isn't enough to lower my score as of yet.
Just take a look at your miles per gallon figure if your car lets you see that and see how good a driver you are. The higher your mpg, the more likely your score will be better for a black box because things like harsh braking, harsh accelerating etc are things that mpg is based on. I managed to get my mpg up from 36.5 to 47.5 and saved about £30 a month in petrol.
As for installation, it isn't fitted anywhere near the engine, it's fitted behind the dash in your electrical system. Bell don't change anything to your premiums unless you go over 100mph (this will invalidate your warranty), they can see you go to work in your car when you said you use it for social only and they can up your premiums if you quoted lesser mileage than you actually do in a year, but if your score is bad, they won't automatically up your premium UNLESS you renew. And you don't have to renew, either. If you don't want the box fitted, go somewhere else. It's designed to lower premiums for new and young drivers, so why would race car drivers and older or more experienced drivers benefit? Simple fact is, if you don't want it, don't get it. People who get it have their premium quoted as less as a BONUS for having it installed. So stop your whining.
As for when you decide you don't want it anymore, it can be disconnected from their end any time. You have to pay to take it out, but it doesn't do anything to your car once the box has been disconnected. And they CANNOT go back on their word. Even if they lied and didn't disconnect it, their is NO WAY they can use the information in any way or to use it against you in the future.
Some people might be "afraid" that this is a tracking system designed to keep tabs on you, but at least their being honest about it. It's the tracking systems that aren't honest you want to watch out for. Plus it's an option service, no-ones making you get one.