This morning Dan Ackerman at CNET published his
review of the Surface Pro 4. At the end of the second page, he compares the specs with other systems
including the Surface Book. (He is probably prepping the Surface Book review as we speak.) In that table he lists the SB i5 processor as a 2.4GHz Intel Core i5-6300U (2 cores).
The good news is that compared to the Surface Pro 3, the faster processor plus the switch to the PCI-e interface for the SSD should be a very noticeable bump in performance.
The less good news is that this is exactly the same processor line used in the SP4. So, without the dedicated graphics option, the performance of both the SB and SP4 should be the same. I was hoping the SB would use one of the quad core chips without the throttling used in the "U" line of Intel Core Processors. The Haswell version of that line was used in the SP3. It is designed to minimize heat and battery drain. This gives you good performance in everyday tasks, but throttles back on jobs like rendering video or compiling software - tasks that try to run the CPU at full capacity for sustained periods of time.
Bottom line: there are very few tasks that would benefit from upgrading to an i7 processor. We saw this on the SP3. Things that would normally benefit from the i7 feature set are the same jobs that hit the heat/battery ceiling.