How much do MX radiators lower water temp?

I am surprised this has no funny or really interesting answers.
measure exhaust gas temp minus cylinder temp? cylinder is being cooled by the coolant .hmmm. you are going to have to sacrifice an engine to get this answer.keep us posted

I think the effect is to keep engine temps more stable and consistent.

No.
If you want to measure how much a radiator cools, you need to plumb in temp sensors before and after the radiator. However the radiators ability to shed heat changes with ambient air temp, humidity, air pressure, and speed.

This is pretty good explanation. I would add the speed equates to cubic ft per minute of air flow through the radiators. Technically this could be boosted with fans for more air flow at slower speed. Also there will be a maximum amount of cooling possible for a given design of radiator in relation to cooling surface area. From experiments with air cooling PC CPUs I found that there is a limit too how many degrees of temperature can be removed from a CPU cooler in relation to ambient air temp. That is to say that how many degrees of heat that can be removed tops out at certain level of cubic feet per minute of air flow over the cooler. Increasing air flow won't change the temp once that threshold of air flow is reached as the cooler can't flow heat any faster. At that point either cooler air must flow over the cooler to lower temps further or a more efficient cooler must be installed of a different design of cooling system needs to be installed for increased cooling capacity. Because there is there is that limit generally there will be an amount of degrees difference between ambient air temp and CPU heat that will be found such that for every degree of ambient air temp change there will be a corresponding CPU temp change. Swap motor for CPU and radiator for CPU cooler in this example and it applies to dirt bike cooling systems as both rely on ambient air temp and cooling capacity surface area of radiators.

I am not sure if all my terms are scientifically right in that explanation but the idea should be spot on.

May as well toss in that radiators or in the case of an air cooled bike the cylinder fins isn't the only form of cooling going on. In regards to carb tune fuel richness/leaness also is part of the cooling. The more fuel/richer the cooler an engine will run. The less fuel/leaner the hotter an engine will run. Engine temp can be manipulated by working with the fuel mix. This manipulation tops out in regards to engine efficiency, cooling system capacity and the variables noted above.

Sensor would be the only way to tell...

From boiling to operating temp of about 170 deg.

Paw Paw