I've been dumping four corners with -4 lines running out of the front port to the back, then tieing banks together with -8 and dumping off the side.
540's with dart blocks, afr heads, and Brodix hv2000 intakes (they have 4 ports stock) - 700hp NA. I'm using a crossover with bypass, but had to restrict the output flow with a reducer to about .200" for each engine to get enough water temperature. It still dumps quite a stream. Using a tstat with 3 .188 holes in it.
Temps are within 5 degrees front and back always.

I've run this combo as is for 13 years without any issues. Just preventatively rebuilt the engines (deglazed cylinders, rings, top ends). Very similar burn front to rear. I sent the heads out to Bob Teague to inspect and he confirmed the valves were in excellent shape for having 700 hours on them and running typical ethanol E10 pump gas.

I'm just looping, with a cross over and Stewart Racing thermostat, it has high flow with holes already in it. I run in pretty cold water in Northern Michigan so I barely hit 160 unless I'm really hammering it. I have lightning headers and the inner and outer jacket have a small gap where the water is introduced to the exhaust flow. I took my sawsall and used a long blade and inserted it in the gap which was a tight fit. I used the blade to open the slot enough so the blade would fit all the way around. I also opened up my water inlets on my lower unit and ported them so they would catch more water at speed and ported my Hardin stainless water pump so it would flow more water. That basically consisted of rounding the square edges in the flow path for less turbulence. I keep an IR temp gun in the boat and after hard runs I pop the hatch and check head temps on the exhaust side and intake. I don't see much variance from front to back, less than 2 degrees, I don't recall the numbers but they were low. All I know is at idle, I am dumping a ton of water out the exhaust and I can run all day at 60mph and not hit 160 degree water temp.