You don't "do" 115 120...
You mean you HEARD that a friend of yours has done that...lol, never admit to breaking the speed limit.
I agree, I think the 300 is a capable machine. I thought for a while I want to upgrade to the 500, but I'm glad I gave it a shot and stuck to the 300. Once you get a feel for it and learn it's peak points, it's awesome and plenty to get by for my needs. I'm not racing anyone, nor do I care to.
There are plenty of seat options to choose from that offer more comfort. There's been lots of threads on that on this forum. You'll have to live with the vibration, it's just the way it is. As for the wind, I find putting a knee out to counter drag really helps keep it in a straight line.