it wont necessarily increase the top speed. thats governed by things like gear ratios. what it will do is make you accelerate towards the top speed ALOT faster.
what car have you got. a normal remap on a GTI will raise the Bhp by around 40hp and the torque by about 70lbft.
Reference insurance id say on average from what people on here have said and what i had myself your looking at around £50 rise in your annual premium provided your with a modification friendly broker such as greenflag, adrian flux, chris knott etc.
Selling a car on isnt to bad, you can reset the car to stock via the tuner, or if you go for a shark performance or bluefin remap you have handset that will revert the car back to the stock setup, and then you can sell on the handset as well to claw back some of the money