[BillA]

not sure I agree with Ben's 'quote' of mine
nor that I understand quite what myv65 is saying
(probably I'm overlooking something ?)

what I can understand is that the valve as shown will permit the manual control of the coolant temp
- but note that with the valve shut the cooling will be 100%,
open any amount it will be something less that that - is too much cooling bad ?

looking at the valve's effect on the flow rate through the wb:
with the valve full open (assuming it is a ball valve with an ID not less that the tubing used), there would be some reduction in the overall resistance . . .
but not really so much as the flow is going through the branches of 2 Ts
(which also add a bit of resistance to the rad circuit as its flow must go through the run of these same 2 Ts)

so the question is actually:
Is the presumed improved convection within the wb due to the slightly greater flow of greater effect than the (CERTAIN as I see it) rise in the coolant temp due to part of the flow not being cooled at all and the other portion being cooled slightly 'better' ?

my experience with flow diversion is that the backpressure the pump 'sees' changes only slightly,
and therefore the flow rate changes only slightly as well

decreasing the flow through the rad is not going to improve performance

EDIT: an additional note
if one wants to 'tune' the CPU cooling system, it is a whole bunch easier to vary the air flow through the rad(s)