I thought I read here that some racers were using a 2 gallon lawn and garden sprayer filled with water to cool the trans and converter between rounds.

There is always this.

Yes, lose converters build more heat.

Thicker trans fluid will tighten the converter, less shear on the fluid inside the converter (IE tighter), is a better coupling between the pump and the turbine in the converter so it generates less heat.

Ringless input shafts (with the exception of the ATI ringless shaft and super pump or super stator in a stock style pump there may be others but ATI is the only ones i've used) reduce the amount of fluid flow in the cooling circuit of a power glide. If you want a simple test, remove the supply line to the cooler and stick it in a bucket and crank the car. There should be a full flow from the hose going into the bucket. Some ringless shafts in PG's severely limit the amount of fluid flowing in the coolant circuit, the flow from the hose will look like a small stream vs a full flowing #6 line when you try the test. You want to check flow here not pressure IMO.

Don't know anything about a turbo 350, but sounds like the additional HP you made caused more slip in the converter or if you put a new converter its less efficient so its got more slip (not a bad thing cause lose converters are usually faster) unless its way lose at the top end then its just generating a bunch of heat and not going anywhere. In either case you need to add cooler capacity. Get a bigger one, get it away from the radiator, and add a separate fan.

Synthetic fluids are potentially better at managing heat, could be due to heat transfer capabilities of the fluid, but in my case i believe it was mostly due to it being a thicker fluid, it tightened the converter flash up by 200 RPM and lowered the top end RPM by 300 in a quarter mile run.