Hi Mralex, the thing is you have good diaphragms if you were to stiffen the springs you would only slow the opening of the slide as for the closing of them that would be mute. The springs are simply a balance between low pressure and high pressure.
You didn't mention your compression results and the reason I asked is as you know the other side of that is expected vacuum, for example are the valve seating are the rings sharp enough to vacuum enough so that the CV carb's can work within there limits.
Now heres one for you, my son and I just did a small road trip and before leaving, he road the Kat 1100 which we have played the carbs many times with different configurations, he was saying that the rpms weren't behaving well and that the milage seemed some what off, but that the power band was strong all the way through! So pull the air box ran the carbs through there paces, heres what I found the number one slide had the lower washer/bushing that the spring seats to incorrectly placed and acted as a shim, like lowering the needle clip or seating the needle high. So what I expected when I saw the gas jetting through at under 3000 was a worn needle or seat that would be what you are refering to with your thought of stiffening the spring seating the needle with more pressure. If you don't see gas jetting out or a "wet ring around the jet at low rpm's then there is no problem there, if you do than check needle for ware or out of round jet!
Tracking down poor milage isn't easy but the compression results are a good place to start, next is valve lash, when that meets the bench mark then you tune the carbs accordingly.
Hope this helps you figure out what is a miss with the bike.
S/G