Just put it on yesterday, so no. I don't think this is a good place to put any heavy objects as it will load the forks, and, as you noted, put a lot of weight up high. I plan on keeping a small waterproof bag/duffle with 1st aid stuff, spare gloves, and a rain jacket that goes over my main jacket.
Yes, that is the one I would buy or I would just make one similar to the ManRack version. I could knock out a ManRack copy for about $5 of material and a couple hours so I will likely go that way. Just didn't want to build it then find out it makes the bike too top heavy. Most of the time I'm carrying a saw I'm on single track so I don't want it to be unmanageable. Right now I'm just using a backpack but it would be nice to put the saw up front.