I am not answering for Terry, as he can do that himself, but I thought I would take a shot...
All filters have a pressure drop. As they collect debris, the pressure drop gets higher. If you look at a filter specs (I do this in my job), they are rated for a given pressure drop at a given flow rate. In addition, a good one has a spring loaded bypass so a clogged filter won't stop the flow entirely. The spring bypass might be rated 3PSID (Pounds per Square Inch Differential). That means when the filter is fully bypassing, the pressure drop across the filter is at least 3 lbs. The poresuure could be 1000PSI in, 997PSI out, a difference of 3PSI) The pumps are rated at a volume/pressure. The pressure is the sum of all resistances in the circuit (Much like electric). The oil passages, orifices, valves, internal filter in the tranny all add up to a pressure drop at a given gpm. Let's say 1000 PSI at 3 gpm (not a rating for our internal tranny pump, but a number to use). If you decrease resistance (clean filter, larger pipes, etc) the pump can deliver 3.2 gpm at 950 PSI. If you increase resisitance, the pump might deliver 1050 PSI at 2.8 gpm. The problem with our cars, is the higher pressure might be good, but the lower flow through the cooler is not good.
In reality, the pump is a fixed displacement pump. There is a pressure regulator that dumps extra oil if the pressure gets high. So while the pump is actually delivering the same GPM at a given rotational speed, the regulator start dumping some of that GPM when the pressure get's higher than it's setting. This is to maintain an even pressure across all RPM input ranges. A low rpm, it is dumping very little. At high RPM, it dumps a lot.
Now I hear your mind thinking, let's up the regulator setting. Good Idea...but, the pump isn't rated for that much higher pressure and the oil pump casing is been known to fail at the pressures it runs now. Since we can't increase the pressure, we really don't want to add any restriction to the present system.
To do this right, you need to redesign the pump housing and gears for more strength at a higher pressure, change the regulator setup, increase some of the passage and tube size. Not so easy. I don't know what Level10 does to improve performance, but is probably along these lines.
FWIW, even a small cooler, which should be a help, can increase the pressure drop and reduce flow.
So that's why you must be careful of the filter. Since I don't know what the flow actually is, and I know that the flow is a little low without an external filter, I would say an external filter will give you cleaner oil at a higher temperature than now. And high temps kill the tranny.
The only way around this might be an external pump, tap into the tranny pan, let the external pump pull oil out of the pan, filter it, and return it to the pan. This would work, but I don't know if it would be really cost effective. I would just change the tranny oil more frequently and put an appropriate sized oil cooler on.
