How much of an improvement was noticed when you guys upgraded to something aftermarket? I'm looking at spending around 400 on a H/U and wondering whether it would be worth it or not - single din + pocket is probably the way to go at this price.

Definitely go aftermarket.

In a previous thread I suggested the Pioneer FH-P5000MP (now available on e-bay for A$350)

I've got it in my TL and it improved the stock speakers by a great deal.

And you can tune the sound much more specifically with this headunit too. It's a double-DIN, you have CD/MP3, tape, and AM/FM radio, and you can also connect an iPod to it easily.

I wholly recommend it!

i've got a kw premium unit, 6 stacker in dash and all....
Has been sitting around for a while, id sell it for a 100 bucks
if it didnt work id be happy to take it back and return your money also