I drove around without a passenger seat for a week in my mk1 1.2, made it more nippy. Completely strip it out, buy 11" steels that are very narrow but have just enough grip and thats as fast as your getting.
My K&N induction kit has slowed my car down. Adding a back box will probably increase back flow and slow ur car down. No point in a remap, you'll still be slow lol
Thats about all you can hope for in terms of low cost etc. What i'm doing is just waiting until those NCB's build up and enjoy a bigger faster car later on.