it kinda depends on what you want man a top mount will have less lag as there is less piping but the downside of it will have more heat soak as it is close to the turbo and downpipe,the front mount will run cooler but will have more lag as will have more piping,again depends what you want as in are you going for more power,looks and what mods have you or want to do in the future,this should help you taken from andy forrest site http://www.andyforrestperformance.co.uk/27032.html
as it says on andy forrest site the v5/v6 top mount will become ineffecient at around 330 bhp and the newage sti is good for over 450 bhp with an appropriately sized turbo