Those stem extenders are generally pretty strong. My folks have them on all of their bikes, and my brother even flipped over the bar on one of them doing something silly at a bike skills park. The bike was not hurt -- can't say the same thing for younger bro. His body healed sooner than his ego did.
Gaarci it sounds like you prefer plenty of standover clearance. I'm the same way -- a frame that's the "proper" fit for me is a little tighter than I prefer on standover, so I generally err on the smaller side if I have the choice. I generally have 6-8" of seat post showing on my bikes. Your saddle definitely looks pretty tall -- it looks like you must have long legs to be able to cycle the crank at that position.