Will an RS3 spoiler fit an audi a3 1.8t?

Considering the 8L never came in RS3 guise and the 8P and 8V cars are totally different bodies with different dimensions, I'll highly doubt it will fit.
 
Unless there is a third party 8L compatible version of the RS3 spoiler I highly doubt a genuine one would fit. Also a genuine one would cost you a fortune! I'd recommend you to get a cheap S3 spoiler off eBay and fit that instead. It's more in keeping with the looks of the 8L shape also and you can get them from as cheap as £10 on eBay! If needed buy the whole tailgate and just get it painted.