Lets take it from the top. Since you know the RS4 ss, from the old days, then you know the Evo and the Drift 3. Not much has changed from that chasis except the motor and slighty better material on the plastics parts and upperdeck.
The Drift 3 moves the motor to the center for more balance, thus helping keep the rear end from swing around (spin-out) which in turn help you hold longer drifts.
The transmission is indeed a single speed. This is also better for RC drifting, as the two-speed transmission could make handle/maintaining a proper more difficult (its doable, just more difficult for a number of reason...the obvious being there is no user feed back to feel the car sliding) Keep in mind, you can upgrade to the 2-speed tranny should you want use your car for racing.
The main chassis is also lighter than the Evo 3. This too, helps initiate and maintain longer drifts.
The real factor is in the tires! When it comes to rc drifting (four wheels at that) there has to be a perfect balance between traction and slippage.
The Drift 3 uses a hard rubber (almost feels like plastic tire) which gives just enough traction for slight steering and straight-line acceloration.this is the equivalent to putting electric tape on our tires like we used to do back in the day. But they last alot longer and look more realistic. As you pointed out, you can easly throw on so foam tires should you want to do some parking lot races, but this car, by no means is meant to be a racer and require some upgrading if you plan to use it that way.
Now if you look at it from the other side of the coin, you could buy the Evo 3, a set of the drift tires and then tune the camber and toe setting untill you get the same thing. HPI just did it for you the other with the Drift 3.
My 2 cents.....