Yes, but the difference is a small value of pounds, and though it will increase the aircraft speed, at high speed the total drag curve will be dominated by the exponentially rising friction drag curve so the increase in speed will be small, in the magnitude of a handful of mph. A disadvantage is that, all else being equal, the higher exhaust velocity engine will burn fuel more rapidly to get the same thrust, so you get a shorter flight or start with more fuel = more weight = slower top speed!
You will certainly get ram effects at 150 - 200mph, but due to model design they will be less than on full size. Our ducts do not provide 100% of their air to the engine and ram it into the engine! Sometimes the engine is not in a duct at all, just sitting open in the fuselage, the airflow into the intake will be fast but will slow down inside the diverging fuselage and the engine itself sits in a much slower airflow. Even fully ducted installations in models still have quite a bit of bypass air around the engine. Engines out in the open may get more ram effect than an engine which is simply sitting unducted in the fuselage but still at these speeds it is not going to make a marked difference.