apparantly its ment to be 33hz but thats a load of ****. yeah, like len said, the danger will be that it is overpowering because its so close, a 10 inch might be your best option as both the size and the noise you want would fit. may not get much passenger legroom though, IIRC mk2 front seats dont go all that far back.
If you want the noise of a sub but want to keep your boot I know someone who is selling some killer underseat custom made builds if you want me to find out more about them for you, such as price and what you will need to power them because I think they are pretty power hungry but I know they sound very good.