hey and welcome to the site mate.
check your booklet and see what fuel it says to use, i dont think the 1.4 needs to use super unleaded so itll be a bit of a waste.
filter and exhaust will help slightly, but you wont get a big difference as its quite a heavy car for the power. your talking you would need to look at headwork be get a really noticable gain.
for the price youd pay for cams, gas flowing the head and a remap to get it all working togeather then you could be looking at around £1500 - 2k. inc labour bear in mind that you would need a custom remap so would be looking at more than a couple of hundred. also remember and think about the insurance gains
considering that selling the 1.4 and buying a 1.9 vrs would be a massive difference after remapping.