The Cup is so much lighter due to no ABS, no spare wheel, removed sound deadening, thinner glass, different rear seat, lightweight alloys, etc...
The 306 Rallye is 52kg lighter, so a decent saving, but remember, they are still a bigger car than a Clio, so were heavier to start with and have not got as much stuff stripped off.
If I was to remove bits from my 306, then I could save even more weight, but Id prefer to have a spare for if/when I get a puncture and I like the idea of ABS just in case!
The 106 Rallyes were actually a LOT lighter than their fully trimmed up counterparts though...
106 Rallye mk1 - 825kg
106 XSi (mk1) - 900kg
106 Rallye mk2 - 865kg
106 GTi (mk2) - 961kg
A saving of 75kg in the mk1 and a massive 96kg in the mk2!!!
However, the 106s were a lot more stripped out than the 306, hence the bigger saving. Basically, the 106 and 306 were not aimed at the same market, so they didnt wanna fully strip the 306. Also, the 106 Rallyes had less power than their fully trimmed brothers, so they needed the massive weight saving to boost performance, wheras the 306 has the same power unit.
At the end of the day, the 306 is still a very quick point to point car!