i also have a full milltek non resonated.
sound perfect, just the way i wanted it to be.
normal when you drive normal, a bit louder with a deep thunder-ish noise when you push it
had the milltek also on the polo before it was mapped.
fuel economy went a bit up but not much.
got it mapped a while ago and fuel economy went down when you drive normal.
but when you floor it, it gets thirsty
it's worth it all the way.
polo's should be made like this leaving the showroom
about Decat, i does make a diffrence, a bit more power and torque is gained without a CAT.
and the sound becomes evil, it will spit fire and give people goosbumps
if it's worth it you should make that out for your own.