Absolutely right, except that steering speed is down to the 'transit time' of the servo. When you see servo specs listed, you'll see torque in ounce-inches, or kilogram-cm, this is a literal description: ounce inches is the number of ounces that servo can lift on a one inch lever. Kilogram centimetres, similarly; how many kilograms it can lift on a one cm lever. You'll also see transit time (and usually a figure in degrees, usually 60*), and this is the time it takes that servo to move from left to right through a 60*arc. Very fast times are like .07sec, medium are around .12 and .18 and over are considered quite slow, and may potentially prevent you from going lock to lock fast enough as you fly through a chicane. Both of these specifications are usually made at 4.8 and at 6v, so be careful you are looking at the 6v readings. You'll normally see 100oz-in@6v, 0.09sec@6v, etc.

Of course these transit times are unloaded, so if you put a load near the limit of the servos torque, the transit times may be reduced a little, and if the servo has only 10oz-in, it will hardly turn the car at all, regardless of transit speed. Therefore we can surmise that most important is always torque. Hence in my post above I said '100oz-in of torque and as fast as you can afford (meaning transit time)'.

Whether it's worth investing in a high power servo for parking lot bashing is a question you can only answer (if you have the money, then I'd say definitely yes, but if you have other priorities for your cash outside the hobby, I'd say maybe not). Using a radio with endpoint adjustment, and setting up a good servo correctly will make an ASTONISHING difference to the way the car drives. You'll notice with your stock servos, that the amount the wheels can turn is quite limited during high speed runs. This is a legacy of the fact they are low torque and the wheels are spinning fast (rotating mass effect). The thing is, to get the best handling from the car, even on parking lots, you will want foam tires for great grip. That's great, but foam tires means even more torque will be needed to steer the car due to the increased traction. At that point a high (or at least high-er) torque servo will certainly be needed.

A lot of people who enjoy the hobby decide fairly early to get a good radio system. The good thing about radios is that they often have the ability to memorise settings for multiple models, so you only ever need to buy one high quality radio. After that, each new car you buy only needs a new receiver. On top of this, a good radio will give you the important adjustments, EPA (servo end points), D/R (Dual rate steering), subtrim and exponential. You will want to buy a good radio whether you end up on a track or not, the only question is when. Anywho... once you get a good radio, it makes sense to get at least one good digital steering servo for the steering, cos as I say, it makes a HUMUNGOUS difference to the way the car drives, especially at speed.