I had an 89 KX500 also. Rode it in the Washington woods.
One ride, I was behind two KDX200s on one of my favorite tight trails.
I caught up to them and rode their tail until the tight single track split
into two single tracks for a 100 feet or so. When I saw the split, I gassed it, and passed the first guy like he was going backwards. A little ways later,
the trail split again, and I zapped that guy with a mere twist of the wrist.
I then pulled away from them and stopped at the end of the single track where it dumped on a road.
They stopped and we chatted. They asked if my KX was a 250. I said "No, its a 500". Their jaws ratcheted down a few notches and they said they
were amazed that I could ride such a beast on those tight trails.
I just told them it was a matter of throttle control. If you pinned it, you were going to be bouncing off the trees. I then mentioned that when a hill comes, or the trail opens up. it is an e-ticket ride when you pin it. They said I came around them really fast. I told them that was only about 1/2 throttle because of the trees.
So my answer is: Get a 500. They are too much fun. Just be easy with your throttle movements until you get used to it.
FYI: At a Honda Demo day in Arizona, Rick Johnson told me that he could always ride a 500 faster than a 250. I said that HE could, since he was a pro, but he said even someone at my level would be faster once I learned throttle control. He was right. I got the KX a little later and I have owned a CR500 and a XR650R since.