AFAIK:
Weight - as previously mentioned, about 70kg less than equivalent GSR.
0-60? Knock a tenth off if it makes you happy, ~5s depending on the level of mechanical sympathy, it's all academic.
Brakes on E1-3 the same as GSR, only 4|PLS| had different brakes.
RS will have a/c (knobs), just not climate control (buttons and VF display).
Foglight not a reliable indicator of RS/GSR, I've seen RSs with factory foglight (in numberplate surround on bootlid) and GSRs without and vice-versa. This varies by evolution.
Crap interior and steel wheels are usually replaced by this stage.
Black (as opposed to body-coloured) door mirrors and door handles are a pretty safe bet for spotting an RS at a glance.
When it all comes down to it, seems Mits had about the most flexible production line going when building Lancers, hardly 2 identical models left the line! Chassis numbers are the best (only?) way of differentiating between the models with 100% reliabilty.