because you spend more time at higher speeds. with quicker acceleration you get to a high speed faster that means you will be absorbing more energy when braking and the brakes will overheat. especially the stock solid disk brakes on a k11. micras are city cars and so they were designed for an average 30mph around town, if you are boosting up to a high speed and braking over and over then it will make a difference.
as for the £4k that i estimated i have spoken with a lot of people who have done turboing on micras, of course it makes a difference if you just dump a turbo on there and be done with it, that will cost you next to nothing, but the greatest thing about a micra is how reliable it is, but it wont be with just a turbo stuck to it. i think 4k is reasonable for a reliable and modest turbo setup, if you want everything done properly and you want it to be reliable. if you can fabricate exhaust manifolds and intake tubing, and you don't want an intercooler etc, then you can do it for cheaper, but is it really worth putting all that effort into having a car with a turbo setup and not have things like an intercooler? if you map the engine (which costs) and then add an intercooler, you have to map again to get the most of it. do it all in one go, do it properly or just put in a bigger engine.
