For what it's worth..........
I ride a variety of terrain out here in Southern California, some resembles, if vaguely the riding area that you posted pictures of. Except there are bushes, and no clear running streams usually in the desert. However, there are many a tight twisty trail out in the desert and the local mountains where we ride during the summer months. And as you will see in an upcoming ride report, I spent some time riding in Idaho and Washington on miles and miles of single track.
And while I have not ridden a 450, my personal prefference is a 550. Why you might ask? I have not found the 550 to have "too much power" in the tight stuff, and while others opinions are that the 450, 380, et. al are more flilckable in the tighter terrain, I have not found my 550 to be non-flickable. It's also pretty cool when you come upon an obstacle that requires lofting the front end, nothing more is usually required than to open the throttle. I have also found that when riding tight trails or open terrain it sure is nice when you come upon a gnarly uphill with a short run, all you have to do is punch off and you have almost instant momentum.
The 550 has a power band that is mellow down low, and builds power as the revs are increased to a great top end hook. If you want instant power, just fan the clutch and you won't be dissappointed.
I rode 250's for years and all my buddies rode open bikes. I always thought a 250 was enough for me until I bought my 94 &1/2 FE501. In the first race I rode it in, I came to an uphill of loose rocks and silty dirt with my fellow competitors scattered all over it. I had to weave my way around the stuck riders, and when I had to slow to get around someone, all I had to do was punch off once around them and I instantly had my momentum back. From that day forward I knew why all friends rode open bikes. They are so much easier to ride.
So that's my opinion, take it for what it's worth.