Actually the way UV maps are used is part of the problem. If you look at M2 or V2 texture maps, you will notice that the eyes represent a very small portion of the overall map. If you want to do a close up of the face and have the eyes look good you need a large texture map just to get the eyes right. The best solution to this problem is not to create extra large UV maps but to use multiple UV maps. When DAZ did V3 they put the eyes on a separate map. (Still I have no idea why we need >1024x1024 maps for eyes). Smart use of multiple UV maps will reduce the need for high res. Unfortunately many designers try to put everything on one map, thus the need for high res maps. One thing that I have done a lot is to scale the texture maps in photoshop. Make a back up of the high res maps incase you need them later. You can make new MAT poses to apply the different res maps. Unless you are doing very close up work, I doubt if you will ever see the difference between a 1kx1k map and a 4kx4k map if the render is 1024x768 or less.