Yes, less weight will increase your rpms. And adjusting the rivet weights in the two different holes will adjust the rpms at different ranges in your powerband. There are so many variables that can change the weight you use like different clutch components, whether you ride on hardpack or boondock or mtn riding, altitude differences, etc.
For example I have an srx that I long tracked to 144. A friend of mine set it up for me where it worked great on hardpack but when I took it boondocking through fields with 1-2 feet of snow in it I was under revving like you are. So I had to lighten up the weight to compensate for it since that is the type of riding I normally do. You will never get it to perform perfectly in all types of riding conditions but you want to set it up for what you normally ride in.
It's pretty tough to test it without snow, even if you had a nice big stretch of grass to try it on right now your sled will be preforming different on grass than snow and even the high temperatures would make a huge difference because you would probably be running rich with your jetting thus not have as much horsepower as when it is cold and you are running at normal conditions. I'd say you have to wait until you are in the proper conditions to really be able to do proper clutching.
If I were you I would pull that middle weight and try it so you can get some kind of a base line to start at and go from there. You can tweak it some after you know where you are sitting at at that point. You can get different size rivets to try at your local Yamaha dealer to do some fine tuning if you have to.
