Are you going to change the charge level of the r134a to give the unit more capacity so you can put more heat into it?
I've always thought with chillers that capacity tends to be more important than getting really low temps, because if you go too low then it will all freeze when it is idling no matter what you put in it and will give you massive condensation issues, and if you are putting CPU(s) and GPU(s) then that totallys quite a bit of heat so you want it to be able to handle that.
G