Ive spoken to a performance
shop near me that my friend owns and he rekons that 18's will fit and should still be able to lower it about 35 - 45 mm. As long as I get the right ofset etc... and I dont go for a tyre to wide the they'll fit so I'm told.
As with the acceleration I think it might kill a bit too much speed thats why I'm not sure.
Those wheels are by an american company called axis, the only uk importer that I've found is Gap security and I think that they're pretty expensive check out the website though there's alot of nice wheels on there.