So 11-chains should be about 28.7 times harder to find than 10-chains. So that number is definitely closer to 30 rather than 10. Sunny King originally assumed that the ratio would be about 32. Also note that the ratio keeps growing as the chain length increases.
I also compared these numbers to some earlier benchmarks. It seems that the ratios are smaller with more optimized code. So my guess is that for GPU miners the ratios may be slightly smaller. (Anyone want to confirm that?)
I have used up to hp11 and rdb's to mine until diff passed ~10.3. I found that the ratio is close to 10 from diff=7 to 8, and from 8 to 9. Since the ratio is highly miner dependent (some may even tune the miner to find more shorter chains to cheat pools), maybe it is best to get the chain length from all blocks for each target length, and calculate the overall ratio for that target length. Then we can find out how the GPU miners changed the ratio.
I'd be interested to see a chart of such ratio as a function of target length.
Anyway I have calculated money supply cap using ratio=30
here. According to it 5 years after release XPM diff won't reach 14, and the total supply is going to be ~55 Millions XPM.