I know there are a few people running Wintsch labs blocks as pure water blocks with great results; unfortunately I don't have any links to provide you with.
You probably answered this but did you follow the suggested inlet/outlet configuration that came with the block? Are you testing this mounted in a case or laying on a table? I personally dropped all the mounting instructions that came with mine and just mounted it directly to the backplate and tossed on some evenly distributed dow corning conformal coating and neoprene to insulate between. So far it's holding -4c stress testing for 24 hours and is looking pretty solid.
Did you get the block directly or used? I got mine used and ended up lapping it due to a couple of defects; this made a huge difference in my temps and EasyPCKits has a GREAT lapping kit which made it easier than expected. Their premium lapping kit costs $13 and ships express for I think $3... http://easypckits.com/
Either way; great work and I hope it works out for you.