Lower Profile Tyres

[pazz]

You will knock the speedo out and then also the tacho (mileage counter etc). You can calculate by how much it will be out using that website which was posted above.

The alternative is to change diameter of the rim, or the tyre width. Either way, it can be dangerous running on low profile tyres. A friend of mine owned a scoobaru with 18's on and he ditched the car into a field. One of his "low-profile" tyres completely ripped off the rim while he was giving it some.

[gazbutS3]

"it can be dangerous running low profile tyres",

what a strange statement, many cars including the GTI come on low profile tyres, not sure they would be fitted if they were in anyway dangerous

[john_o]

altering the profile in combination with tread width but keeping within a reasonable percentage of the OE rolling radius is in theory fine 
In combination with a reduced ride height it may indeed give you the look you are after.
Be wary though moving away from 'known' combos (17's 45profile , 18's 40profile , 19's 35profile) may give you ride/wear and handling issues you wouldnt expect. Rim protection will also be reduced.

If you want to do it fine, go for it  Personally I would be looking at others ways to enhance your car but sometimes its good to be different......
One of the GTI's facets is great agility/handling which you dont want to spoil , but you never know it may improve to your liking 

note that jonnyc is moving away from std sizes to fit certain rims and tyres

[pazz]

"it can be dangerous running low profile tyres",

what a strange statement, many cars including the GTI come on low profile tyres, not sure they would be fitted if they were in anyway dangerous

Strange? I suppose I should of been more specific. Try "Lower profile tyres".

For example you have the Monza GTi setup of 18" rim on a 225/40/18. Then fit 225/35/18 and the side wall is now only 35% of the nominal width = 78.75mm instead of the stock 90mm. Have a play with this: http://www.miata.net/garage/tirecalc.html.

Using the above calculator to save me having to do the numbers, you'd need a 255/35/18 to produce the same size of sidewall.

The use of "low profile tyres" is a bit of a given these days on performance cars. Better side-wall technology has made low profile tyres possible. But manufacturer's choose a tyre setup very carefully, moving away from a stock tyre setup just for the sake of it looking better is just asking for problems in my opinion.

My friend with the scooby fitted aftermarket wheels and didnt realise he needed to look into what the best tyre/rim combo should of been for the rim width/diameter.

As JOHN_O has already mentioned the "known" combos are known for a reason. Because they give safe side-wall's (amongst other things).