It would stand to reason that there is some sort of additional restriction on the 200 motors intake beyond what the 250 has... not just the venturi, but after it. Sort of like the difference between an eheim 1260 and 1262 (or what companies like deltec do which is buy the 1260 and drill out the inlet so its the same as a 1262... making it the same pump). Otherwise, yes, they would be the identical motors.
That being said, they are identical motors from the handling capacity of the actual impeller, volute, and electrical end of things. This can actually be a bad thing, since restricting a pump is one thing, if done within reason, but restricting a pump, and then making it a needlewheel kills its power factor. Even though the RMS wattage might drop, if the pump's spin is slipping forward more due to the lowered resistance (lighter material) in the volute, the VA, or real power of the motor can slip up. This is why needlewheels run hot, and can even trigger an internal fuse. The solution is to use a larger volute than if the pump was just for water. Is there any way to put a different volute, maybe from a larger pump, onto the dart motor? Thats the way to do it... make the pump more of a flow-biased pump, and open up any restrictions. The motor will cool better then as well, and be less likely to be damaged from running in such a manner... either that, or you need some sort of phase/power factor correction circuitry going on. You dont even need to change impellers... the larger volute will allow for more air handling yet with very little additional water (sometimes less). Ahhh... why couldnt they all just be DC motors... lol.
Force feeding is possible, but only if you dont go much above the original intake of the venturi. If you raise the air too much in the venturi, you will be replacing water with air in the venturi, and the venturi will act as a valve restricting water even more. For the reasons stated in the last paragraph, this can be damaging to the motor and pump (although more so with mag-drive pumps which need the water flow for cooling as well). The motor will slip even more out of phase, and its efficiency will drop.
If you are going to force feed, you need to remove the venturi and plumb straight pipe onto the intake of the pump. You might be able to still restrict the water a little bit (depends on how much air), so you could put a valve on the inlet before you 'T' in the air, but to control the water flow, you can just add more air... more air means less water in the end. This is perhaps the best way to run these motors, because then they are less restricted. The only problem comes with the output bubbles... too much air or even too much waterflow and you pass the bubbles w/o being blended as well. The solution to this is next, but it could be considered a solution in itself.
These needlewheels arent all that great from what I have seen. But they are needlewheels after all so they have to be made a specific way sometimes. To make them more effective, the needles should be smaller, and denser. The air gets chopped up finer, and this mix can then be moved easier by the motor (multiple smaller faces of pins rather than a few larger blades). If you put air into a non-needlewheel, it will handle it, but in much lower volume. When you add more air, the blades on the impeller hit a large pocket of air as it comes in and cant really do much with it other than deal with the pressure drop. The idea of a needlewheel is partly to blend and chop, but most of that actually happens as the air/water spin around the volute before making it out. The other main part is to provide a more diffused interface/surface between the mechanical impeller and the 2-phase mixture entering the pump. By using smaller and smaller surfaces (pins), the mix gets chopped up, but is also spreads out the mechanical force of the impeller across multiple smaller surfaces which can easier deal with hitting either air or water as they spin because they are smaller and that many more of them than 4-8 large blades. There are many ways to put it, but in the end, the finer and denser the needles, the better the performance. Which leads us to mesh.
You dont have to remove the pins, but considering the design I have seen of the ReefFlo needlewheel, I would suggest interlacing some enkamat 7010 into the needlewheels. It will increase the performance of the pump by alot. If you force feed, the pump will be able to handle more air and chop it up finer. If you just run it as is... it will still improve the flow through the pump (not to mention increase the efficiency and lower how hot the motor runs). You will have to hold it on somehow... perhaps some fishing line threaded through some holes on the impeller. You dont have to go nuts with it, as you could actually make the impeller too heavy for startup (although I doubt it with this size motor), and there is a point where the density of the mesh will no longer help, but just take up volume in the impeller well that would otherwise be going towards air and water (flow). But it would help the pump 'bite' into the air & water more. You might even need to enlarge any restrictions on the intake to compensate (even the venturi) since the mesh is going to try and grab more air and water all together.
