Yield is mostly about the number of plants you can fit! Secondly how thick you can grow them.
In direct real world comparisons CFLs yield about 75% of HPS, +/- 25%. It really depends on how they're used. If you were to just replace HPS directly with CFLs(same basic fixture location/volume ... one direction light, from the top down, for instance) HPS always shows a significant advantage, CFL's would be closer to 50-60%.
CFLs require you surround the plant for optimal yield, and when doing this the results can be pretty similar. Here is where you can get above 75% compared to similar HPS, and if done properly, probably with less power(total). As CFL lets you get light more or less exactly where it needs to be.