Well firstly, then you say "daily or weekly average" you have to narrow that done into a specific project since every project gives different credit amounts. Currently what you have is nearly the equivalent of saying "how much fps does this graphic card give in games", it is too broad of a question.
Here is a comparison of the 2600K and 2500K on Intel's website: http://ark.intel.com/Compare.aspx?ids=52214,52210
Notice that the 2600K has more cache, and has hyperthreading capability. As well, the integrated graphics has a higher max speed. Once you have ignored these things, the chips are the same, with the 2600K having a higher multiplier. The i7 2600K would no doubt be much better for BOINC since you have 8 threads to work with.
As well, comparisons for BOINC on CPU is nonexistent on professional reviews and extremely hard to find on online communities like these due to the amount of projects, and applications, and CPUs out there. It's just too hard to get a proper test bed for these things. Folding@Home fares much better (I remember seeing a TomsHardware review of the Radeon HD 6800 series that included F@H).
So in the end, despite them not being available, there must be some members on our BOINC community that have Sandy Bridge and hopefully they can spare some time to publish some of their results. As for now, if you can afford it, get a 2600K. If you do want to wait however, do keep in mind that the only high Sandy Bridge CPUs to be released is a chip similar to the 2600K that will be on LGA2011 and then the Extreme line of Sandy's (6core 12thread and a 8core 16thread. These may be in the $1000 range since they are the next generation 980X and 990X). Then in 2012, there are rumors that Ivy Bridge will be coming out which is a whole other barrel of fun...