just to share some thoughts, it is understandable adding fsb or rsb will translate into less chasis flexing which can help in handling. But i think its still need some tolerable flex to accomodate b road condition compare to racetrack. Torsion beam axles@twist beam@also known as semi independant setup. As the name suggest, it did twist. Imho, adding too thick stab bar, rsb did alter the flex n cause it become too stiff. I think the original bending n torsion rigidity on saga already stiff to its engine limits n beyond it actually. Unless u plug in 200hp engine in it, the standard still can hold it.
If want better traction, get better tyre, if want less body roll, switch to sport spring originally design for blm or flx which is rare, or change to better strut, maybe adjustable. I think changing rear stab bar or rsb is the last resort.
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When original satria neo came out, r3 add fsb n rsb becoz the original mitsu chasis has too much flex which needed to stiffen. While satria neo r3 mk1 based on satria neo no need to add coz it already stiff enough to do the job,unless change to high power engine. By adding fsb n rsb,it will add more weight n put more stress on suspension setup. By the way, if u look at satria neo r3 mk1, they used 17 inch rim, while for satria neo r3 mk2, using 16 inch rim. This is due satria neo r3 mk2 is based on satria neo cps which having changes to its setup where the front track is narrower while rear track is wider compare to original setup.
Every setup has its limit in accepting rims n tyres size. Same as flx n flx se. Eventhough the spring n strut is the same, but there are some tweaking to flx se being done to accomodate 15 inch rim n tyre size. Because using negative chamber setup, using diff rims n tyre profile than original may cause uneven wear coz its never tested on the setup. Remember, the chamber is fixed, so, cannot adjust it to accomodate the diff rim sizs n tyre profile. When being said negative, it means total chamber. Yup,it is perfect to have the same chamber left n right, but there is some manufacturing tolerance when mass produce it unless it is too much, then it is faulty.
And just some advice, product for the mass generally for general used, not track day used. Not denying torsion beam axle chamber can change due to massive force(over limit), but if being set to a sport car like megane, then it suits it purpose. But independant setup is another way around where easier to adjust, but if broken, it cost a bomb.
If want to adjust on torsion beam, need to use machine press n it takes hours to change its chamber slightly coz if press to hard, can ruin the beam.
Just sharing some thought only. Not arguing :-).

Thanks again for the informative post. So the RSB is actually detrimental to the health of the torsion beam? Stabilizer bar yes, but RSB too?