Nope, not possible to tighten down further. Trust me I tried till my hands hurt. I will make do with the washers that are in there, because I got a reply back from Verdes stating they don't provide washers and to use the tightest springs on the tension I want and it will wear in.
Uhmm not if I put them on the tension that I would like.
The tension that it is at where the screws already start grinding the plastic of the centerpieces is still too tight, and if I put things on the tension I actually WANT to have it (where it is now.. finally, roughly, with two washers) the centerpieces would not even be able to move anymore and be locked in place by the screws and eventually they would break off if I were to try cubing regardless.
One thing: be very careful on the lubing. This cube does NOT respond well to lubing the way you'd normally lube a 3x3. I don't overdo it or anything, I KNOW how to properly lube a cube; I have had compliments from speedcubers much faster than I am who got a lubed cube from me. This v-cube slows down significantly with a normal lube job, it needs VERY little.
I just took mine apart yesterday and took the lube out, and it sped up like a madman again. I am steering clear from lube save for literally a drop or two every now and then. Fair warning in any case.