I see what you mean, running some quick estimates Id say the water block has a surface area for heat transfer of around 480cm^2. Its a nice block design, but that's not a huge number.

For reference, the Coolit products have a hot side surface area of 565cm^2 (except the Boreas which I think doubles this).

Also you may get some losses from laminar flow but I don't think that's a major problem in that design.