I also have 1.4s, the one to avoid if wise at time of purchase! Optimax definitely makes a difference, i can tesify to that, give slightly more torque low down where it lacks originally.
Haven't got round to an induction kit or replacement filter yet so cant vouch for them.
have done the exhaust route though. replaced the backbox with a remus twin pipe two years ago. Added some sound and slightly more torque again, mostly low down and slightly more free at high speeds as well. This box replace the original backbox only though, not the whole section. This whole section went next and i wanted to keep the remus box so had a powerflow custom centre section fabricated. Added 100% more noise when acceleration low down and slightly more torque again. Not Neck Snapping but certainely noticeable.