Gary,
a) where in Vic are you from? I'm in (or will be in a couple of months) from Geelong
b) I here what you're saying on the 520. I like you, in some ways am pretty used to big bore bikes. Husky 610's, KX & CR 500's. There is nothing the world like the feeling one of these beasts gives :) I'm in the U.S. at the moment and up until 2 weeks ago had a 380 MXC (not availible in Aust. but is probably more comparable to the SX than EXC) The 380 is a FAST, yet a sweet bike to ride. It is not noticibly slower than a 500, and in fact I think it accelerates harder than a cr500 (still wouldn't touch my KX). The handling however is no comparison. The 380 is a modern designed, slim, light bike that is a 380 in 250 motor / frame. IMO it's an ideal combo for someone who wants the big bore 2 stroke power yet have a light handling bike.
A couple of weeks ago, I got to ride a friends 2001 520 EXC at an MX track for about 20 minutes, straight after stepping off my 380. My first / immediate impression was that it was slower, lacked response, was lazy, heavy and didn't turn as sharp. I was boardering on dissapionted. Then after a few minutes I suddenly realised that I was jumping further, clearing a few more obsticles and didn't have as much of a "death grip" If someone had of been timing me I would say I'd been 10% faster straight off the bat.
Now, my intention when I get home in a couple of months is to get a 520. I think they suit my riding style and type of riding (mostly bush) they give me a higher degree of confidence to twist long and hard.
My reccommedation would be to give it a little more time. It's a completely different ball game on a 4-stroke, particularly with a long 2 STK history. Neither bike does everything perfectly, nothing will. It's just a matter of fit between bike-rider-terrain.
However, the short answer to your question if your asking if the 380 is a good bike? YES Fantastic, Fast and exciting.
p.s. If you decide to sell, give me a bell, I'll be looking for one withing 2 minutes of getting out of the airport!