Good grief ? It sure didn't take long to make those changes to my bike. I did a little bit the night before each ride and a month later it was done. Plus, I've outlined what to do, so there is very little guess work.
It may not have taken long, it just seems like A LOT looking at the big picture. And certain things like removing the tank mount (which you mentioned needed reinforcement) seem like a move in the wrong direction. Yeah you shaved a bit of weight, but was it really worth the ~1/3 of a pound? I'm not knocking you for your effort, I think you've done a great job and are on the right track for sure, I just don't know if I'd go about it exactly the same way as you did.
Furthermore, its way less work to lighten a WR than it is to get a YZ ready for woods. The YZ suspension is going to need a complete revalve and I'd need a WR transmission for sure.
If you do buy a YZ, be sure to reroute the valve cover breather into the air box so that if you ford a deep stream and stall it, it doesn't suck water into the engine.
Admittedly I have spent WAY too much time and money getting my YZ set up for woods. My old 00 YZ 426F was easy... add handguards, a spark arrestor, and radiator braces, lower the gearing, and twist the throttle. This 06 has been an exercise in futility, and even after 2 revalves I'm still not happy with the suspension. It's WAY better than it was though.
FWIW I've never had issues with either of my YZ's sucking water up the crankcase breather. I think it would take a certain set of circumstances and everything to line up just right for it to happen. Not that it can't, I just don't think its a huge concern.
I'd love to hear what you would do. I'm always looking for ways to improve my WR.
If it could be done I'm sure someone would have attempted it by now, but I was thinking it would be interesting to see if you could get the 12+ WR EFI system operational on the 06-09 YZ chassis (or pre EFI WR chassis). It's probably far more work and $$$ than it's worth, but it would be fun to try. The big hurdle (I think) would be getting the fuel pump in the tank.