Put a k&n 57i on my Fiesta ST.
Hasn't made a massive difference to be honest with you, does sound pretty nice now though. Have noticed it idles so much better than before. Does seem to pull better in 5th gear compared to my brother's ST. Before, he would pull away...
Only £80 investment, would do it again as I've been able to compare mine with a standard example.
Edit: Also adapted it this weekend and moved it lower, towards the grille. Could notice heatsoak before, will see how I get on with it now.