I have started removing the springs as a standard procedure whenever the underlevers are out of the piano. Steinway has not used underlever springs in their new pianos in quite some time now, and it does make a difference in the feel of the piano. For the better, IMHO. This is in line with my aim to have the pianos I service come as close to current production pianos as possible. I do, more often than not, get permission from the owner.
I seem to recall Del Fandrich contending that springs do little to speed damping. Mass is much more effective. (Please correct me here, Del, if I am in error.) Carefully regulated, with good felt (yes, the Steinway felt is very good-their engineers looked very carefully at it in the not-too-distant past), the dampers work very well without springs. And the touch is noticeably lighter and smoother.
Note that in very old systems, the underlevers are narrow, and therefore have less mass. Adding a little may be a good idea, and is actually not that hard. Buy leads or get some lead wire (fishing suppliers). Forstner bit and a squeeze or small hammer blow.
A recent M system (same for S, M, L, and O) has 28 underlevers with 3 leads (about 12 grams), 17 with 2 leads (10 g), 17 with 1 lead (8 g), and 5 with none (5 g). The weight measurements are approximate. I invite someone out there with better measuring tools to take more data. I measured with the tray in its rest position and then seeing what it takes to lift the underlever at the very end. FYI...
Oh, so I have several sets in small plastic bags if you want some.