tbh any speaker update or stereo just wont cut it in a mk2.
door speakers are 5.25.
MK4 polo dash tweeters are a good upgrade on the cheap.
You need them low's in your life! I dont understand how people can drive with no lows!

Im running high end Phoenix Gold 5.25 components up front..still sounds sh!te.
Infinity 4x6's and a pioneer headunit without the fancy gizmo's.
In boot -Alpine Digital class D amp and a 10'' JL audio that kicks ass!
Although it can make your nose itch its toned right down for quality and not a headache!

A decent sound system transforms a long journey!
I'd buy components over coaxels and look into sound deadning the doors first!