Marvin:
If your block is a single inlet and outlet on the sides, then long channels like that can result in laminar flow. My guess is that the cross channels would stop a laminar flow regime from developing and improve performance.
Re: surface area. If cutting channels in blocks reduced surface area then an unworked hunk of copper would be the ideal
I think if you measure and run some calculations that the difference in surface area will not be that great (you expose 4 sides on the resulting "pins" rather than the 2 on the existing fins and so you aren't necessarily making or losing surface area but merely repositioning it). Obviously the smaller your cross channels the greater the resulting surface area.