In-car screens are small. The clarity and resolution of a screen with RCA inputs will not suffice; you won't be able to read anything. That's why computer monitors don't come with RCA inputs. You need a screen with VGA input.
To your second question, there isn't a simple answer. How quickly DashCommand updates data will be determined by the number of parameters being displayed/logged, the speed of your computer, the kind of parameters you are logging, the cable you're using, etc. Obviously there are people out there that are satisfied with it, but I don't want to spend $90 (software+cable: they don't support the 1.3 tactrix) on something I already suspect will be a failure.
However, there are some really cool things about DashCommand. Obviously one big thing is you can customize your gauges. You can have multiple gauge layouts (called "dashboards") and flip through them with a simple swipe of the screen. You can set alarms, customize DashCommand to do certain things automatically like start logging when you start the car. It is also the only OBDII software I have seen so far with the ability to integrate aftermarket sensors (PLX). Unfortunately, one PLX sensor+module is usually over $100, so just four sensors would cost you as much as another carputer. Plus you would have a bunch of stupid metal boxes lying around. DashCommand also has very good customer support, a mildly active forum, and good documentation to help you set it up.
EvoScan, on the other hand, uses the SSM protocol. Unfortunately, it is buggy, and their customer service sucks. They have absolutely no documentation, no directions or anything. I can't figure out how to get it to start logging automatically, and I feel like that is the most important feature of such a product. It doesn't always read correctly, and even when it does, it won't do it when you put something on a gauge. For example, it will read rpm properly, but if you embed it into a gauge layout, it just reads "0000" the whole time. It also doesn't have nearly as many useful loggable parameters as RomRaider, and has no aftermarket sensor support.
One thing that is nice about EvoScan, however, is you can change "formula" of any parameter so you can make it display any range of numbers you want. This would be helpful if, say, you want convert your mph reading to kph (hypothetical). With a USDM WRX, you could also get rid of any useless sensors (EGT, rear O2), wire in aftermarket sensors, and change the formulas to read properly (if you know what you're doing).