You may need a wind screen. I've seen guys take a small conventional wind screen and mount it on the bars of their trailies. Seems to work well enough. I saw one particular owner who had some kind of quick release set up which allowed the screen to be removed when he went into the trails. Of course he had to carry a bag on back to hold it.
6'3" eh? You wont have any problems on the 650. I've cut my seat down on mine. I just don't relish the idea of coming to a stop in the forest, putting a foot down and finding a 2 foot depression, then falling over. I'm 5' 11' and the XR650L is bordering on too tall for me. I'm going to see how much a local dealer is going to suck from my pockets on a trade in for a DRZ400. This one is used and the enduro model, but comes with extra tires, larger tank and the Baja Kit which permits it to be road legal, with brake light switch, indicators etc.
The XR650L has such a good suspension that I've been on trails that a 650 shouldnt be on. But the type of terrain here is very wet, tight, lots of steep hills and rocks. The 400 is physically smaller which will help a lot. Too bad that this model of DRZ is kick start only.
Does your 650 have a larger tank on it? Mine came with a monster Acerbis tank that must of held close to 6 gallons

I traded that tank for a Clarke 3.5 gallon tank which is more reasonable for the type of riding I do. The weight of that full Acerbis tank when riding in tight stuff was too much. Good for Baja and other open places though.
Re the R6. One of the Journos from Cycle Canada mag rode an R6 across Canada and said he was suprised at how comfortable he was. I doubt very much that he was your heighth though.
I think this is the url..
www.thumpertalk.com
There's also a company that sells nothing but goodies for the XR series bikes. Based in the USA. I think it's
www.xrsonly.com