Can't recommend it enough - if you are thinking about doing it then just go for it. I paid £200 to remap the TDI Polo from 100bhp to ~130bhp, knocked an easy 2 seconds off my 30-70mph time, and I can still squeeze 70mpg with gentle driving and 205 width tyres. There are no downsides so long as you tell the insurers and don't floor it in 1st - the torque will tend to rip the car to bits if you're a heavy starter......