the rear luggage bar acts like a rear brace hence the oversized bolts on it.
i have had my rear seats out for 4 months solid 800-1000miles a month work / personal driving. initially when you remove them you get a better sound especially bouncing off the limiter and it feels quicker. i weighed up the cup seats at 20kg inc luggage bar seen as the cup seats are the most basic either your 1.2 will have same or better so that figure should stand for you.
however having driven very fast over country roads i can confirm the handling gets worse unless your car is lowered and i also suggest keeping luggage bar in (5kg`s of the weight) or getting rear strutbrace instead (ktec carbon fiber perhaps) my cup sat higher at the back and looked a bit silly.
why does the handling get worse ? its Front engined and front wheel drive and there is no seats in the back of a car with all the stuff in the frontwithout back seats weight distribution (although not exellent anyway) gets too uneven at speed. the short wheelbase of the car means that at the best of times a clio will bounce / lift over undulating roads at speed and without back seats and bar in place the back end slides left to right.
i had a go against my dad`s audi 3.0tdi the otherday he said while i was in front reaching over 1/00 on a road with a bad negative camber the tail end was all over the place which i felt also too much bounce. im not saying your 1.2 is going to pelt down the road as fast as my (at the time 1000kg 175bhp cup) but i know have them in again and its miles more stable at speed it corners better and doesnt pitch as much it also feels stiffer at the back and of course if much quieter (even though i dont mind noise hence nearly 4,000miles 4months of no back seats and a hillpower fitted) i have to say its a nice change to have it all back together. also makes the car look cheap without them in.
if your car is lowered, and you keep luggage bar in then expect 15kg weight saving lots more tyre noise and a small perceptable increase in accelaration, however its easy to remove seats and so you may aswell give it a go to try it out and put them back in when you have finished, i would bother re-carpeting etc.. until you know you can live with the extra lift and noise.
hope this helps, just my experience, im sure if my car were lowered a bit more at the back it wouldnt be so bad but thats just the experience i have had.
i await a lenghy discusion about luggage bars.. lol
phill