Vermin,
I will attempt to answer this for Gary as I know he is going away and may not be available to answer for 10 days or so.
You need to square the resolution of the encoders because they work in 2 axis.
The encoders have 8,192 steps in each axis, which when matrixed gives you 8,192 x 8192 = 67,108,864 possible permutations. The 10,000 step encoders give you 10,000x10,000 = 100,000,000 possible permutations. Therefore the 10,000 step encoders give you an extra 32,891,136 permutations over the 8,192 step encoders permutations of 67,108,864.
The % increase in total resolution is 32,891,136/67,108,864
or an increase of 49.01 %
I think I went pretty close to collecting the chocolates
CS-John B